Semiconductor device

ABSTRACT

A novel semiconductor device in which a metal film containing copper (Cu) is used for a wiring, a signal line, or the like in a transistor including an oxide semiconductor film is provided. The semiconductor device includes an oxide semiconductor film having conductivity on an insulating surface and a conductive film in contact with the oxide semiconductor film having conductivity. The conductive film includes a Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/546,443, filed Nov. 18, 2014, now pending, which claims the benefitof foreign priority applications filed in Japan as Serial No.2013-248284 on Nov. 29, 2013, and Serial No. 2014-038615 on Feb. 28,2014, all of which are incorporated by reference.

TECHNICAL FIELD

One embodiment of the present invention relates to a semiconductordevice and a display device each including an oxide semiconductor.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, alight-emitting device, a power storage device, a storage device, amethod for driving any of them, and a method for manufacturing any ofthem.

BACKGROUND ART

There is a trend in a display device using a transistor (e.g., a liquidcrystal panel and an organic EL panel) toward a larger screen. As thescreen size becomes larger, in the case of a display device using anactive element such as a transistor, a voltage applied to an elementvaries depending on the position of a wiring which is connected to theelement due to wiring resistance, which cause a problem of deteriorationof display quality such as display unevenness and a defect in grayscale.

Conventionally, an aluminum film has been widely used as a material usedfor the wiring, the signal line, or the like; moreover, research anddevelopment of using a copper (Cu) film as a material is extensivelyconducted to further reduce resistance. However, a copper (Cu) film isdisadvantageous in that adhesion thereof to a base film is poor and thatcharacteristics of a transistor easily deteriorate due to diffusion ofcopper in the copper film into a semiconductor film of the transistor.Note that a silicon-based semiconductor material is widely known as amaterial for a semiconductor thin film applicable to a transistor, andas another material, an oxide semiconductor has attracted attention (seePatent Document 1).

In addition, as an ohmic electrode formed over a semiconductor filmcontaining an oxide semiconductor material including indium, a Cu—Mnalloy has been disclosed (see Patent Document 2).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] PCT International Publication No. 2012/002573

DISCLOSURE OF INVENTION

Regarding a transistor in which a silicon-based semiconductor materialis used for a semiconductor film, research and development have beenextensively conducted on a structure in which a copper film is used fora wiring, a signal line, or the like while copper in the copper film isnot diffused into a semiconductor film. However, there has been aproblem in that the structure and its manufacturing method are not yetoptimized for a transistor using an oxide semiconductor film.

Furthermore, a transistor using an oxide semiconductor film in which acopper film is used for a wiring, a signal line, or the like and abarrier film is used to suppress diffusion of copper in the copper filmhas had a problem in that electrical characteristics of the oxidesemiconductor film deteriorate, the number of masks for the transistorusing the oxide semiconductor film is increased, or the manufacturingcost of the transistor using the oxide semiconductor film is increased.

In view of the above problems, an object of one embodiment of thepresent invention is to provide a novel semiconductor device in which ametal film containing copper (Cu) is used for a wiring, a signal line,or the like in a transistor using an oxide semiconductor film. Anotherobject of one embodiment of the present invention is to provide a methodfor manufacturing a semiconductor device in which a metal filmcontaining copper (Cu) is used for a wiring, a signal line, or the likein a transistor using an oxide semiconductor film. Another object of oneembodiment of the present invention is to provide a novel semiconductordevice in which a metal film containing copper (Cu) in a transistorusing an oxide semiconductor film has a favorable shape. Another objectof one embodiment of the present invention is to provide a novelsemiconductor device or a method for manufacturing the novelsemiconductor device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Objects other than the above objectswill be apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention is a semiconductor deviceincluding an oxide semiconductor film having conductivity on aninsulating surface and a first conductive film in contact with the oxidesemiconductor film having conductivity. The first conductive filmincludes a Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti).

Another embodiment of the present invention is a semiconductor deviceincluding an oxide semiconductor film having conductivity on aninsulating surface and a first conductive film in contact with the oxidesemiconductor film having conductivity. The hydrogen concentration inthe oxide semiconductor film having conductivity is higher than or equalto 8×10¹⁹ atoms/cm³. The first conductive film includes a Cu—X alloyfilm (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti).

Another embodiment of the present invention is a semiconductor deviceincluding an oxide semiconductor film having conductivity on aninsulating surface and a first conductive film in contact with the oxidesemiconductor film having conductivity. The resistivity of the oxidesemiconductor film having conductivity is higher than or equal to 1×10⁻³Ωcm and lower than 1×10⁴Ωcm. The first conductive film includes a Cu—Xalloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti).

Note that the first conductive film may be a pair of conductive films,and the oxide semiconductor film having conductivity and the pair ofconductive films in contact with the oxide semiconductor film havingconductivity may serve as a resistor.

Alternatively, the semiconductor device of one embodiment of the presentinvention includes an insulating film in contact with the oxidesemiconductor film having conductivity and the first conductive film,and a second conductive film in contact with the insulating film andoverlapping with the oxide semiconductor film having conductivity withthe insulating film provided therebetween. The oxide semiconductor filmhaving conductivity, the first conductive film, the insulating film, andthe second conductive film may serve as a capacitor. Note that theinsulating film may include a nitride insulating film.

The first conductive film includes a Cu—Mn alloy film. Alternatively,the first conductive film is a stack of a Cu—Mn alloy film and a Cu filmover the Cu—Mn alloy film. Alternatively, the first conductive film is astack of a first Cu—Mn alloy film, a Cu film over the first Cu—Mn alloyfilm, and a second Cu—Mn alloy film over the Cu film.

A coating film including a compound containing X may be provided on theouter periphery of the first conductive film. In the case where thefirst conductive film includes a Cu—Mn alloy film, manganese oxide maybe provided on the outer periphery of the first conductive film.

The oxide semiconductor film having conductivity includes a crystalpart, and a c-axis of the crystal part may be parallel to a normalvector of the surface where the oxide semiconductor film is formed.

The oxide semiconductor film having conductivity may include an In-M-Znoxide (M is Al, Ga, Y, Zr, Sn, La, Ce, or Nd).

According to one embodiment of the present invention, a novelsemiconductor device in which a metal film containing copper is used fora wiring, a signal line, or the like in a transistor using an oxidesemiconductor film can be provided. According to another embodiment ofthe present invention, a method for manufacturing a semiconductor devicein which a metal film containing copper is used for a wiring, a signalline, or the like in a transistor using an oxide semiconductor film canbe provided. According to another embodiment of the present invention, anovel semiconductor device in which a shape of a metal film containingcopper is favorable in a transistor using an oxide semiconductor filmcan be provided. According to another embodiment of the presentinvention, a novel semiconductor device of which productivity isimproved can be provided. According to another embodiment of the presentinvention, a novel semiconductor device or a method for manufacturingthe novel semiconductor device can be provided.

Note that the description of these effects does not disturb theexistence of other effects. In one embodiment of the present invention,there is no need to obtain all the effects. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1E are cross-sectional views illustrating embodiments of asemiconductor device of the present invention;

FIGS. 2A to 2D are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device of the presentinvention;

FIGS. 3A to 3D are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device of the presentinvention;

FIGS. 4A to 4C are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device of the presentinvention;

FIGS. 5A to 5F are cross-sectional views illustrating embodiments of asemiconductor device of the present invention;

FIGS. 6A to 6C are cross-sectional views illustrating embodiments of asemiconductor device of the present invention;

FIGS. 7A to 7D are cross-sectional views illustrating embodiments of asemiconductor device of the present invention;

FIGS. 8A and 8B are circuit diagrams each showing one embodiment of asemiconductor device of the present invention;

FIGS. 9A and 9B are a top view and a cross-sectional view illustratingone embodiment of a semiconductor device of the present invention;

FIGS. 10A and 10B are cross-sectional views illustrating embodiments ofa semiconductor device of the present invention;

FIGS. 11A to 11C are cross-sectional views illustrating embodiments of asemiconductor device of the present invention;

FIGS. 12A to 12C are cross-sectional views illustrating embodiments of asemiconductor device of the present invention;

FIGS. 13A and 13B are cross-sectional views illustrating embodiments ofa semiconductor device of the present invention;

FIGS. 14A to 14C are cross-sectional views illustrating embodiments of asemiconductor device of the present invention;

FIGS. 15A to 15C are a block diagram and circuit diagrams illustratingone embodiment of a display device;

FIG. 16 is a top view illustrating one embodiment of a display device;

FIG. 17 is a cross-sectional view illustrating one embodiment of adisplay device;

FIGS. 18A to 18D are cross-sectional views illustrating one embodimentof a method for manufacturing a display device;

FIGS. 19A to 19C are cross-sectional views illustrating one embodimentof a method for manufacturing a display device;

FIGS. 20A to 20C are cross-sectional views illustrating one embodimentof a method for manufacturing a display device;

FIGS. 21A and 21B are cross-sectional views illustrating one embodimentof a method for manufacturing a display device;

FIG. 22 is a cross-sectional view illustrating one embodiment of adisplay device;

FIG. 23 is a cross-sectional view illustrating one embodiment of adisplay device;

FIG. 24 is a cross-sectional view illustrating one embodiment of adisplay device;

FIG. 25 is a cross-sectional view illustrating one embodiment of adisplay device;

FIGS. 26A and 26B are cross-sectional views each illustrating oneembodiment of a transistor;

FIG. 27 is a top view illustrating one embodiment of a display device;

FIG. 28 is a cross-sectional view illustrating one embodiment of adisplay device;

FIGS. 29A to 29C are cross-sectional views illustrating one embodimentof a method for manufacturing a display device;

FIGS. 30A to 30C are cross-sectional views illustrating one embodimentof a method for manufacturing a display device;

FIG. 31 is a cross-sectional view illustrating one embodiment of adisplay device;

FIG. 32 is a cross-sectional view illustrating one embodiment of adisplay device;

FIGS. 33A to 33C are cross-sectional views illustrating one embodimentof a method for manufacturing a display device;

FIGS. 34A and 34B are cross-sectional views each illustrating oneembodiment of a display device;

FIG. 35 is a cross-sectional view illustrating one embodiment of adisplay device;

FIG. 36 is a cross-sectional view illustrating one embodiment of adisplay device;

FIGS. 37A to 37D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS;

FIGS. 38A to 38D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS;

FIGS. 39A to 39C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD;

FIGS. 40A and 40B show electron diffraction patterns of a CAAC-OS;

FIG. 41 shows a change of crystal parts of an In—Ga—Zn oxide owing toelectron irradiation;

FIGS. 42A and 42B are schematic views showing deposition models of aCAAC-OS and an nc-OS;

FIGS. 43A to 43C show an InGaZnO₄ crystal and a pellet;

FIGS. 44A to 44D are schematic views illustrating a deposition model ofa CAAC-OS;

FIGS. 45A and 45B illustrate an InGaZnO₄ crystal;

FIGS. 46A and 46B show a structure and the like of InGaZnO₄ beforecollision of an atom;

FIGS. 47A and 47B show a structure and the like of InGaZnO₄ aftercollision of an atom;

FIGS. 48A and 48B show trajectories of atoms after collision of atoms;

FIGS. 49A and 49B are cross-sectional HAADF-STEM images of a CAAC-OS anda target;

FIG. 50 shows temperature dependence of resistivity of an oxidesemiconductor film;

FIG. 51 illustrates a display module;

FIGS. 52A to 52E are each an external view of an electronic device ofone embodiment; and

FIGS. 53A and 53B show a STEM image of Sample and a result of EDXanalysis.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described below with reference to drawings. However,the embodiments can be implemented with various modes. It will bereadily appreciated by those skilled in the art that modes and detailscan be changed in various ways without departing from the spirit andscope of the present invention. Thus, the present invention should notbe interpreted as being limited to the following description of theembodiments.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, suchas “over” “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. The positional relation between components is changed asappropriate in accordance with a direction in which each component isdescribed. Thus, the positional relation is not limited to thatdescribed with a term used in this specification and can be explainedwith another term as appropriate depending on the situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode layer) and a source (a sourceterminal, a source region, or a source electrode layer), and current canflow through the drain, the channel region, and the source. Note that inthis specification and the like, a channel region refers to a regionthrough which current mainly flows.

Furthermore, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an “object having any electric function” aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Examples of an “objecthaving any electric function” are a switching element such as atransistor, a resistor, an inductor, a capacitor, and elements with avariety of functions as well as an electrode and a wiring.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention is described with reference to FIGS. 1A to 1E, FIGS.2A to 2D, FIGS. 3A to 3D, FIGS. 4A to 4C, FIGS. 5A to 5F, and FIGS. 6Ato 6C. In this embodiment, a structure of an oxide semiconductor filmhaving conductivity and a conductive film in contact with the oxidesemiconductor film and a manufacturing method thereof are described.Here, the oxide semiconductor film having conductivity serves as anelectrode or a wiring.

FIGS. 1A to 1E are cross-sectional views of an oxide semiconductor filmhaving conductivity and a conductive film in contact with the oxidesemiconductor film which are included in a semiconductor device.

In FIG. 1A, an insulating film 153, an oxide semiconductor film 155 bhaving conductivity over the insulating film 153, and a conductive film159 in contact with the oxide semiconductor film 155 b havingconductivity are formed over a substrate 151.

Furthermore, as illustrated in FIG. 1B, an insulating film 157 may beformed over the insulating film 153, the oxide semiconductor film 155 bhaving conductivity, and the conductive film 159.

Alternatively, as illustrated in FIG. 1C, the oxide semiconductor film155 b having conductivity may be formed over an insulating film 157 a.In this case, an insulating film 153 a can be provided over the oxidesemiconductor film 155 b having conductivity and the conductive film159.

The oxide semiconductor film 155 b having conductivity is typicallyformed of a metal oxide film such as an In—Ga oxide film, an In—Zn oxidefilm, or an In—M—Zn oxide film (M is Al, Ga, Y, Zr, Sn, La, Ce, or Nd).Note that the oxide semiconductor film 155 b having conductivity has alight-transmitting property.

In the case where the oxide semiconductor film 155 b having conductivitycontains an In—M—Zn oxide film, the proportions of In and M whensummation of In and M is assumed to be 100 atomic % are preferably asfollows: the atomic percentage of In is greater than 25 atomic % and theatomic percentage of M is less than 75 atomic %, or further preferably,the atomic percentage of In is greater than 34 atomic % and the atomicpercentage of M is less than 66 atomic %.

The energy gap of the oxide semiconductor film 155 b having conductivityis 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV ormore.

The thickness of the oxide semiconductor film 155 b having conductivityis greater than or equal to 3 nm and less than or equal to 200 nm,preferably greater than or equal to 3 nm and less than or equal to 100nm, further preferably greater than or equal to 3 nm and less than orequal to 50 nm.

In the case where the oxide semiconductor film 155 b having conductivityis an In—M-Zn oxide film (M is Al, Ga, Y, Zr, Sn, La, Ce, or Nd), it ispreferable that the atomic ratio of metal elements of a sputteringtarget used for forming the In—M—Zn oxide film satisfy In≧M and Zn≧M. Asthe atomic ratio of metal elements of such a sputtering target,In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=2:1:1.5, In:M:Zn=2:1:2.3,In:M:Zn=2:1:3, In:M:Zn=3:1:2, or the like is preferable. Note that theproportion of each metal element in the atomic ratio of the formed oxidesemiconductor film 155 b having conductivity varies within a range of±40% of that in the above atomic ratio of the sputtering target as anerror.

The oxide semiconductor film 155 b having conductivity may have anon-single-crystal structure, for example. The non-single crystalstructure includes a c-axis aligned crystalline oxide semiconductor(CAAC-OS) described later, a polycrystalline structure, amicrocrystalline structure described later, and an amorphous structure,for example. Among the non-single crystal structures, the amorphousstructure has the highest density of defect levels, whereas the CAAC-OShas the lowest density of defect levels.

Note that the oxide semiconductor film 155 b having conductivity may bea mixed film including two or more of the following: a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure. The mixed film has asingle-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Furthermore, the mixed film has a stacked-layer structure of two or moreof a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline singlelayer structure, a CAAC-OS region, and a region having a single-crystalstructure in some cases.

The insulating film 157 and the insulating film 157 a are preferablyformed of a film containing hydrogen, typically, a silicon nitride filmcontaining hydrogen. When the insulating films 157 and 157 a in contactwith an oxide semiconductor film contain hydrogen, the hydrogen issupplied to the oxide semiconductor film, so that the oxidesemiconductor film 155 b having conductivity can be formed.

The oxide semiconductor film 155 b having conductivity includes animpurity. Hydrogen is given as an example of the impurity included inthe oxide semiconductor film 155 b having conductivity. Instead ofhydrogen, as the impurity, boron, phosphorus, nitrogen, tin, antimony, arare gas element, alkali metal, alkaline earth metal, or the like may beincluded.

The hydrogen concentration in the oxide semiconductor film 155 b havingconductivity is higher than or equal to 8×10¹⁹ atoms/cm³, preferablyhigher than or equal to 1×10²⁰ atoms/cm³, further preferably higher thanor equal to 5×10²⁰ atoms/cm³. The hydrogen concentration in the oxidesemiconductor film 155 b having conductivity is lower than or equal to20 atomic %, preferably lower than or equal to 1×10²² atoms/cm³. Notethat the concentration of hydrogen in the oxide semiconductor film 155 bis measured by secondary ion mass spectrometry (SIMS) or hydrogenforward scattering (HFS).

Including defects and impurities, the oxide semiconductor film 155 bhaving conductivity exhibits conductivity. The resistivity of the oxidesemiconductor film 155 b having conductivity is preferably higher thanor equal to 1×10⁻³ cm and lower than 1×10⁴Ωcm, further preferably higherthan or equal to 1×10⁻³ cm and lower than 1×10⁻¹ Ωcm.

The oxide semiconductor film 155 b having conductivity includes defectsin addition to impurities. The oxide semiconductor film 155 b havingconductivity is typically a film in which defects are generated byreleasing oxygen by heat treatment in vacuum in the formation process, afilm in which defects are generated by adding a rare gas, or a film inwhich defects are generated by plasma exposure in the deposition processor the etching process of the conductive film 159. As an example of thedefect included in the oxide semiconductor film 155 b havingconductivity, an oxygen vacancy is given.

When hydrogen is added to an oxide semiconductor including oxygenvacancies, hydrogen enters oxygen vacancies and forms a donor level inthe vicinity of the conduction band. As a result, the conductivity ofthe oxide semiconductor is increased, so that the oxide semiconductorbecomes a conductor. An oxide semiconductor having become a conductorcan be referred to as an oxide conductor. That is, the oxidesemiconductor film 155 b having conductivity can be formed of an oxideconductor film. Oxide semiconductors generally have a visible lighttransmitting property because of their large energy gap. An oxideconductor is an oxide semiconductor having a donor level in the vicinityof the conduction band. Therefore, the influence of absorption due tothe donor level is small, and an oxide conductor has a visible lighttransmitting property comparable to that of an oxide semiconductor.

The conductive film 159 preferably includes at least a Cu—X alloy film(X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) (hereinafter, simply referredto as Cu—X alloy film), and for example, the conductive film 159preferably has a single-layer structure of the Cu—X alloy film or astacked-layer structure including the Cu—X alloy film. As thestacked-layer structure including the Cu—X alloy film, a stacked-layerstructure of the Cu—X alloy film and a conductive film including alow-resistance material such as copper (Cu), aluminum (Al), gold (Au),or silver (Ag), an alloy thereof, or a compound containing any of thesematerials as a main component (hereinafter referred to as a conductivefilm including a low-resistance material) is given.

Here, the conductive film 159 has a stacked-layer structure of aconductive film 159 a in contact with the oxide semiconductor film 155 bhaving conductivity and a conductive film 159 b in contact with theconductive film 159 a. Furthermore, the Cu—X alloy film is used as theconductive film 159 a and the conductive film including a low-resistancematerial is used as the conductive film 159 b.

The conductive film 159 also serves as a lead wiring or the like. Theconductive film 159 includes the conductive film 159 a using the Cu—Xalloy film and the conductive film 159 b using the conductive filmincluding a low-resistance material, whereby even in the case where alarge substrate is used as the substrate 151, a semiconductor device inwhich wiring delay is suppressed can be manufactured.

The conductive film 159 including the Cu—X alloy film is formed over theoxide semiconductor film 155 b having conductivity, whereby the adhesionbetween the oxide semiconductor film 155 b having conductivity and theconductive film 159 can be increased and the contact resistancetherebetween can be reduced.

Here, FIG. 1D shows an enlarged view of a region where the oxidesemiconductor film 155 b having conductivity is in contact with theconductive film 159. When the Cu—X alloy film is used as the conductivefilm 159 a in contact with the oxide semiconductor film 155 b havingconductivity, a coating film 156 is formed at an interface between theoxide semiconductor film 155 b having conductivity and the conductivefilm 159 in some cases. The coating film 156 is formed using a compoundincluding X. The compound including X is formed by reaction between X inthe Cu—X alloy film included in the conductive film 159 and an elementincluded in the oxide semiconductor film 155 b having conductivity orthe insulating film 157. As the compound including X, oxide including X,nitride including X, silicide including X, carbide including X, and thelike are given. As examples of the oxide including X, X oxide, In—Xoxide, Ga—X oxide, In—Ga—X oxide, In—Ga—Zn—X oxide, and the like aregiven. By forming the coating film 156 serving as a blocking filmagainst Cu, entry of Cu in the Cu—X alloy film into the oxidesemiconductor film 155 b having conductivity can be suppressed.

As an example of the conductive film 159 a, a Cu—Mn alloy film is used,whereby the adhesion between the conductive film 159 a and theunderlying oxide semiconductor film 155 b having conductivity can beincreased. Furthermore, by using the Cu—Mn alloy film, a favorable ohmiccontact can be obtained between the conductive film 159 and the oxidesemiconductor film 155 b having conductivity.

As a specific example, the coating film 156 is formed in the followingmanner in some cases: after the formation of the Cu—Mn alloy film, byheat treatment at a temperature higher than or equal to 150° C. andlower than or equal to 450° C., preferably at a temperature higher thanor equal to 250° C. and lower than or equal to 350° C. or by forming theinsulating film 157 while being heated, Mn in the Cu—Mn alloy film issegregated at the interface between the oxide semiconductor film 155 bhaving conductivity and the conductive film 159 a. The coating film 156can include Mn oxide formed by oxidation of the Mn or In—Mn oxide, Ga—Mnoxide, In—Ga—Mn oxide, In—Ga—Zn—Mn oxide, or the like, which is formedby reaction between the segregated Mn and a constituent element in theoxide semiconductor film 155 b having conductivity. With the coatingfilm 156, the adhesion between the oxide semiconductor film 155 b havingconductivity and the conductive film 159 a is improved. Furthermore,with the segregation of Mn in the Cu—Mn alloy film, part of the Cu—Mnalloy film becomes a pure Cu film, so that the conductive film 159 a canobtain high conductivity.

Alternatively, as illustrated in FIG. 1E, a coating film 156 a is formedon at least one of the bottom surface, side surface, and top surface ofthe conductive film 159, preferably on the outer periphery of theconductive film 159 in some cases. The coating film 156 a is formedusing a compound including X. The compound including X is formed byreaction between X in the Cu—X alloy film included in the conductivefilm 159 and an element included in the oxide semiconductor film 155 bhaving conductivity or the insulating film 157. As the compoundincluding X, oxide including X, nitride including X, silicide includingX, carbide including X, and the like are given.

In the case where an oxide insulating film is formed as the insulatingfilm 157, in a region where the coating film 156 a is in contact withthe conductive film 159 b, an oxide of a low-resistance material isformed. Note that X in the Cu—X alloy film is included in the regionwhere the coating film 156 a is in contact with the conductive film 159b in some cases. This is probably due to an attachment of a residuegenerated in the etching of the conductive film 159 a, the attachment ofthe residue in the formation of the insulating film 157, the attachmentof the residue at the heat treatment, or the like. Furthermore, X in theCu—X alloy film is oxidized to oxide in some cases.

For example, a copper (Cu) film is preferably used as the conductivefilm 159 b, because the thickness of the conductive film 159 b can beincreased to improve the conductivity of the conductive film 159. Here,the copper (Cu) film refers to pure copper (Cu), and the purity ispreferably 99% or higher. Note that the pure copper (Cu) may include animpurity element at several percent.

The conductive film 159 includes the Cu—X alloy film, whereby asemiconductor device in which entry of the copper (Cu) into the oxidesemiconductor film 155 b having conductivity is suppressed and a wiringhas high conductivity can be obtained.

As the substrate 151, a variety of substrates can be used withoutparticular limitation. Examples of the substrate include a semiconductorsubstrate (e.g., a single crystal substrate or a silicon substrate), asilicon on insulator (SOI) substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a metal substrate, a stainless steelsubstrate, a substrate including stainless steel foil, a tungstensubstrate, a substrate including tungsten foil, a flexible substrate, anattachment film, paper including a fibrous material, and a base materialfilm. As an example of a glass substrate, a barium borosilicate glasssubstrate, an aluminoborosilicate glass substrate, a soda lime glasssubstrate, or the like can be given. Examples of a flexible substrate,an attachment film, a base material film, and the like are as follows:plastic typified by polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), and polyether sulfone (PES); a synthetic resin suchas acrylic; polypropylene; polyester; polyvinyl fluoride; polyvinylchloride; polyamide; polyimide; aramid; epoxy; an inorganic vapordeposition film; and paper. Specifically, the use of semiconductorsubstrates, single crystal substrates, SOI substrates, or the likeenables the manufacture of small-sized transistors with a smallvariation in characteristics, size, shape, or the like and with highcurrent capability. A circuit using such transistors achieves lowerpower consumption of the circuit or higher integration of the circuit.

Furthermore, a flexible substrate may be used as the substrate 151, anda semiconductor element may be formed directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate 151 and the semiconductor element. The separation layer can beused when part or the whole of a semiconductor element formed over theseparation layer is separated from the substrate 151 and transferredonto another substrate. In such a case, the semiconductor element can betransferred onto a substrate having low heat resistance or a flexiblesubstrate as well. As the above separation layer, a stack includinginorganic films, such as a tungsten film and a silicon oxide film, or anorganic resin film of polyimide or the like formed over a substrate canbe used, for example.

Examples of a substrate to which a transistor is transferred include, inaddition to the above-described substrates over which transistors can beformed, a paper substrate, a cellophane substrate, an aramid filmsubstrate, a polyimide film substrate, a stone substrate, a woodsubstrate, a cloth substrate (including a natural fiber (e.g., silk,cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, orpolyester), a regenerated fiber (e.g., acetate, cupra, rayon, orregenerated polyester), or the like), a leather substrate, a rubbersubstrate, and the like. The use of such a substrate enables formationof a transistor with excellent properties, a transistor with low powerconsumption, or a device with high durability, high heat resistance, ora reduction in weight or thickness.

As the insulating films 153 and 153 a, a single layer or a stacked layerincluding an oxide insulating film such as a silicon oxide film, asilicon oxynitride film, an aluminum oxide film, a hafnium oxide film, agallium oxide film, or a Ga—Zn-based metal oxide film may be used.Alternatively, the insulating films 153 and 153 a may be formed using ahigh-k material such as hafnium silicate (HfSiO_(x)), hafnium silicateto which nitrogen is added (HfSi_(x)O_(y)N_(z)), hafnium aluminate towhich nitrogen is added (HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttriumoxide. Note that in this specification, “silicon oxynitride film” refersto a film that contains more oxygen than nitrogen, and “silicon nitrideoxide film” refers to a film that contains more nitrogen than oxygen.

Alternatively, the insulating films 153 and 153 a can be formed using anitride insulating film such as a silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, or an aluminum nitrideoxide film.

<Formation Method 1 of Oxide Semiconductor Film 155 b HavingConductivity and Conductive Film 159>

First of all, a method for forming the oxide semiconductor film 155 bhaving conductivity and the conductive film 159, which are illustratedin FIG. 1A, is described with reference to FIGS. 2A to 2D.

First, the substrate 151 is prepared. Here, a glass substrate is used asthe substrate 151.

As illustrated in FIG. 2A, the insulating film 153 is formed over thesubstrate 151, and an oxide semiconductor film 155 is formed over theinsulating film 153. Then, a rare gas 154 such as helium, neon, argon,krypton, or xenon is added to the oxide semiconductor film 155.

The insulating film 153 can be formed by a sputtering method, a CVDmethod, a vacuum evaporation method, a pulsed laser deposition (PLD)method, a thermal CVD method, or the like.

A formation method of the oxide semiconductor film 155 is describedbelow.

An oxide semiconductor film is formed by a sputtering method, a coatingmethod, a pulsed laser deposition method, a laser ablation method, athermal CVD method, or the like. Then, by forming a mask over the oxidesemiconductor film through a photolithography process and etching theoxide semiconductor film with the mask, the oxide semiconductor film 155can be formed.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and an oxygen gas is used as appropriate. In thecase of using the mixed gas of a rare gas and an oxygen gas, theproportion of an oxygen gas to a rare gas is preferably increased.

Furthermore, a target may be appropriately selected in accordance withthe composition of the oxide semiconductor film to be formed.

For example, in the case where the oxide semiconductor film is formed bya sputtering method at a substrate temperature higher than or equal to150° C. and lower than or equal to 750° C., preferably higher than orequal to 150° C. and lower than or equal to 450° C., more preferablyhigher than or equal to 200° C. and lower than or equal to 350° C., theoxide semiconductor film can be a CAAC-OS film.

For the deposition of the CAAC-OS film as the oxide semiconductor film,the following conditions are preferably used.

By suppressing entry of impurities into the CAAC-OS film during thedeposition, the crystal state can be prevented from being broken by theimpurities. For example, the impurity concentration (e.g., hydrogen,water, carbon dioxide, and nitrogen) which exist in the depositionchamber may be reduced. Furthermore, the impurity concentration in adeposition gas may be reduced. Specifically, a deposition gas whose dewpoint is −80° C. or lower, preferably −100° C. or lower is used.

In the case where an oxide semiconductor film, e.g., an In—Ga—Zn—O filmis formed using a deposition apparatus employing ALD, an In(CH₃)₃ gasand an O₃ gas are sequentially introduced plural times to form an In—Olayer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at a time to form aGaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas are introduced at atime to form a ZnO layer. Note that the order of these layers is notlimited to this example. A mixed compound layer such as an In—Ga—Olayer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing ofthese gases. Note that although an H₂O gas which is obtained by bubblingwith an inert gas such as Ar may be used instead of an O₃ gas, it ispreferable to use an O₃ gas that does not contain H. Instead of anIn(CH₃)₃ gas, an In(C₂H₅)₃ may be used. Instead of a Ga(CH₃)₃ gas, aGa(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gas may be used.

After that, hydrogen, water, and the like may be released from the oxidesemiconductor film 155 by heat treatment to reduce at least the hydrogenconcentration in the oxide semiconductor film 155. By the heattreatment, oxygen is released from the oxide semiconductor film 155, sothat defects can be formed. As a result, variation in hydrogenconcentration in the oxide semiconductor film 155 b formed later can bereduced. The heat treatment is performed typically at a temperaturehigher than or equal to 250° C. and lower than or equal to 650° C.,preferably higher than or equal to 300° C. and lower than or equal to500° C. The heat treatment is performed typically at a temperaturehigher than or equal to 300° C. and lower than or equal to 400° C.,preferably higher than or equal to 320° C. and lower than or equal to370° C., whereby warp or shrinking of a large-sized substrate can bereduced and yield can be improved.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature of higher than or equal to the strainpoint of the substrate if the heating time is short. Thus, the heattreatment time can be shortened and warp of the substrate during theheat treatment can be reduced, which is particularly preferable in alarge-sized substrate.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas(argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like.

As the rare gas 154, helium, neon, argon, xenon, krypton, or the likecan be used as appropriate. Furthermore, as methods for adding the raregas 154 to the oxide semiconductor film 155, a doping method, an ionimplantation method, and the like are given. Alternatively, the rare gas154 can be added to the oxide semiconductor film 155 by exposing theoxide semiconductor film 155 to plasma including the rare gas 154.

As a result, as illustrated in FIG. 2B, an oxide semiconductor film 155a including defects can be formed.

Then, the oxide semiconductor film 155 a including defects is heated inan atmosphere including impurities. The heat treatment is performed inan atmosphere including one or more of hydrogen, nitrogen, water vapor,and the like as the atmosphere including impurities.

Alternatively, after the surface of the oxide semiconductor film 155 aincluding defects is exposed to a solution including boron, phosphorus,alkali metal, alkaline earth metal, or the like heat treatment isperformed.

The heat treatment is preferably performed under a condition forsupplying impurities to the oxide semiconductor film, and typicallyperformed at a heating temperature higher than or equal to 250° C. andlower than or equal to 350° C. By performing heat treatment at 350° C.or lower, impurities can be supplied to the oxide semiconductor filmwhile the release of the impurities from the oxide semiconductor film isminimized. Note that the heat treatment is preferably performed under apressure higher than or equal to 0.1 Pa, further preferably higher thanor equal to 0.1 Pa and lower than or equal to 101325 Pa, still furtherpreferably higher than or equal to 1 Pa and lower than or equal to 133Pa.

As a result, as illustrated in FIG. 2C, the oxide semiconductor film 155b having conductivity can be formed. The oxide semiconductor film 155 bhaving conductivity includes defects and impurities. By the effect ofthe defects and the impurities, the conductivity of the oxidesemiconductor film 155 b having conductivity is increased as compared tothat of the oxide semiconductor film 155. As an example of the action ofdefects and impurities, hydrogen enters an oxygen vacancy, whereby anelectron serving as a carrier is generated. Alternatively, bonding ofpart of hydrogen to oxygen bonded to a metal atom causes generation ofan electron serving as a carrier. By these actions, the conductivity ofthe oxide semiconductor film is increased. As a result, the oxidesemiconductor film 155 b having conductivity serves as an electrode or awiring. Furthermore, the oxide semiconductor film 155 b havingconductivity has a light-transmitting property. Thus, alight-transmitting electrode or a light-transmitting wiring can beformed.

Note that the resistivity of the oxide semiconductor film 155 b havingconductivity is higher than that of the conductive film 159. Thus, as alead wiring, the conductive film 159 is preferably in contact with theoxide semiconductor film 155 b.

Next, as illustrated in FIG. 2D, the conductive film 159 is formed overthe oxide semiconductor film 155 b having conductivity. Here, after astack of the Cu—X alloy film and the conductive film including alow-resistance material is formed, a mask is formed over the conductivefilm including a low-resistance material by a photolithography processand the Cu—X alloy film and the conductive film including alow-resistance material are etched using the mask, whereby theconductive film 159 in which the conductive film 159 a formed of theCu—X alloy film and the conductive film 159 b formed of the conductivefilm including a low-resistance material are stacked can be formed.

As a method for etching the Cu—X alloy film and the conductive filmincluding a low-resistance material, a dry etching method or a wetetching method can be used as appropriate. In the case where a copper(Cu) film is used as the conductive film including a low-resistancematerial, a wet etching method is preferably used. The Cu—X alloy filmcan be etched by a wet etching method; thus, when the Cu—X alloy filmand the copper (Cu) film are stacked, the conductive film 159 in whichthe conductive film 159 a formed of the Cu—X alloy film and theconductive film 159 b formed of the conductive film including alow-resistance material are stacked can be formed in a single wetetching step. As an etchant used in the wet etching method, an etchantcontaining an organic acid solution and hydrogen peroxide water, or thelike is used.

Through the above steps, the oxide semiconductor film havingconductivity and the conductive film in contact with the oxidesemiconductor film having conductivity can be formed.

<Formation Method 2 of Oxide Semiconductor Film 155 b HavingConductivity and Conductive Film 159>

A formation method of the oxide semiconductor film 155 b havingconductivity which is different from the method in FIGS. 2A to 2D isdescribed with reference to FIGS. 3A to 3D.

As illustrated in FIG. 3A, the insulating film 153 is formed over thesubstrate 151, and the oxide semiconductor film 155 is formed over theinsulating film 153. Then, heat treatment is performed in vacuum. Byperforming heat treatment in vacuum, oxygen is released from the oxidesemiconductor film 155, so that the oxide semiconductor film 155 aincluding defects can be obtained as illustrated in FIG. 3B. Note that atypical example of the defects included in the oxide semiconductor film155 a is oxygen vacancies.

The heat treatment is preferably performed under a condition forreleasing oxygen from the oxide semiconductor film, and typicallyperformed at a temperature higher than or equal to 350° C. and lowerthan or equal to 800° C., preferably higher than or equal to 450° C. andlower than or equal to 800° C. By performing heat treatment at 350° C.or higher, oxygen is released from the oxide semiconductor film. Inaddition, by performing heat treatment at 800° C. or lower, oxygen canbe released from the oxide semiconductor film while the crystalstructure of the oxide semiconductor film is maintained. Moreover,heating is preferably performed in vacuum, typically under a pressurehigher than or equal to 1×10⁻⁷ Pa and lower than or equal to 10 Pa,preferably higher than or equal to 1×10⁻⁷ Pa and lower than or equal to1 Pa, further preferably higher than or equal to 1×10⁻⁷ Pa and lowerthan or equal to 1E⁻¹ Pa.

Next, by a method similar to that in FIG. 2B, the oxide semiconductorfilm 155 a including defects is heated in an atmosphere includingimpurities. The heat treatment is performed in an atmosphere includingone or more of hydrogen, nitrogen, water vapor, and the like as theatmosphere including impurities.

Alternatively, after the surface of the oxide semiconductor film 155 aincluding defects is exposed to a solution including boron, phosphorus,alkali metal, or alkaline earth metal, heat treatment is performed.

As a result, as illustrated in FIG. 3C, the oxide semiconductor film 155b having conductivity can be formed.

Next, by a method similar to that in FIG. 2D, the conductive film 159can be formed over the oxide semiconductor film 155 b havingconductivity (see FIG. 3D).

<Formation Method 3 of Oxide Semiconductor Film 155 b HavingConductivity and Conductive Film 159>

A formation method of the oxide semiconductor film 155 b havingconductivity which is different from the methods in FIGS. 2A to 2D andFIGS. 3A to 3D is described with reference to FIGS. 4A to 4C.

As illustrated in FIG. 4A, after the insulating film 153 is formed overthe substrate 151, the oxide semiconductor film 155 is formed over theinsulating film 153.

Next, by a method similar to that in FIG. 2D, the conductive film 159 isformed over the oxide semiconductor film 155 (see FIG. 4B). Here, as theconductive film 159, the conductive film 159 a and the conductive film159 b are formed.

Next, the insulating film 157 including hydrogen is formed over theinsulating film 153, the oxide semiconductor film 155, and theconductive film 159. The insulating film 157 is formed by a sputteringmethod, a plasma CVD method, or the like. The insulating film 157 may beformed while being heated. Alternatively, heat treatment may beperformed after the insulating film 157 is formed.

By using a sputtering method, a plasma CVD method, or the like as aformation method of the insulating film 157, the oxide semiconductorfilm 155 is damaged and defects are generated. Furthermore, theinsulating film 157 is formed while heating or heat treatment isperformed after the insulating film 157 is formed, whereby hydrogenincluded in the insulating film 157 moves to the oxide semiconductorfilm 155. As a result, as illustrated in FIG. 4C, the oxidesemiconductor film 155 b having conductivity can be formed. By theaction of defects and impurities, the conductivity of the oxidesemiconductor film 155 b having conductivity is increased as compared tothat of the oxide semiconductor film 155. Thus, the oxide semiconductorfilm 155 b having conductivity serves as an electrode or a wiring.

Modification Example 1

Modification examples of the conductive film 159 are described withreference to FIGS. 5A to 5F. Here, modification examples of theconductive film 159 in FIG. 1B are shown; however, the modificationexamples can be used in the conductive film 159 in FIGS. 1A and 1C asappropriate.

As illustrated in FIG. 5A, the conductive film 159 a can be formed of asingle layer of the Cu—X alloy film over the oxide semiconductor film155 b having conductivity.

Alternatively, as illustrated in FIG. 5B, the conductive film 159 can beformed over the oxide semiconductor film 155 b having conductivity bystacking the conductive film 159 a formed of the Cu—X alloy film, theconductive film 159 b formed of the conductive film including alow-resistance material, and a conductive film 159 c formed of the Cu—Xalloy film.

When the conductive film 159 includes the conductive film 159 c formedof the Cu—X alloy film over the conductive film 159 b formed of theconductive film including a low-resistance material, the conductive film159 c formed of the Cu—X alloy film serves as a protective film of theconductive film 159 b including a low-resistance material; thus, thereaction of the conductive film 159 b including a low-resistancematerial in the formation of the insulating film 157 can be prevented.

Alternatively, as illustrated in FIGS. 5C and 5D, the oxidesemiconductor film 155 b having conductivity may be formed over theinsulating film 157 a formed of a film including hydrogen. In this case,the insulating film 153 a can be provided over the oxide semiconductorfilm 155 b having conductivity and the conductive film 159.

Next, FIGS. 5E and 5F show enlarged views of regions where the oxidesemiconductor film 155 b having conductivity is in contact with theconductive film 159 and the conductive film 159 a respectively. Asillustrated in FIG. 5E, a coating film 156 b is formed on at least oneof the bottom surface, side surface, and top surface of the conductivefilm 159 a, preferably on the outer periphery of the conductive film 159a in some cases. The coating film 156 b is formed using a compoundincluding X. The compound including X is formed by reaction between X inthe Cu—X alloy film included in the conductive film 159 a and an elementincluded in the oxide semiconductor film 155 b having conductivity orthe insulating film 157. As the compound including X, oxide including X,nitride including X, silicide including X, carbide including X, and thelike are given.

In the case where a Cu—Mn alloy film is used as the Cu—X alloy film, asan example of the coating film 156 b, a manganese oxide film is formed.

Alternatively, as illustrated in FIG. 5F, a coating film 156 c is formedon at least one of the bottom surface, side surface, and top surface ofthe conductive film 159, preferably on the outer periphery of theconductive film 159 in some cases. The coating film 156 c is formedusing a compound including X. The compound including X is formed byreaction between X in the Cu—X alloy film included in the conductivefilm 159 and an element included in the oxide semiconductor film 155 bhaving conductivity or the insulating film 157. In a region where thecoating film 156 c is in contact with the conductive film 159 b, anoxide of the low-resistance material is formed. Furthermore, X in theCu—X alloy film is included in the region where the coating film 156 cis in contact with the conductive film 159 b in some cases. This isprobably due to an attachment of a residue generated in the etching ofthe conductive film 159 a or the conductive film 159 c, the attachmentof the residue in the formation of the insulating film 157, theattachment of the residue at the heat treatment, or the like.Furthermore, X in the Cu—X alloy film is oxidized to oxide in somecases. Thus, in the case where a Cu—Mn alloy film is used as theconductive film 159 b, as an example of the coating film 156 c, amanganese oxide film is formed.

Modification Example 2

Here, modification examples of the oxide semiconductor film havingconductivity and the conductive film are described with reference toFIGS. 6A to 6C.

In FIG. 6A, a single layer of the conductive film 159 a formed of theCu—X alloy film is provided between the insulating film 153 and theoxide semiconductor film 155 b having conductivity.

Alternatively, as illustrated in FIG. 6B, the conductive film 159 havinga two-layer structure is provided between the insulating film 153 andthe oxide semiconductor film 155 b having conductivity. The conductivefilm 159 is formed by stacking the conductive film 159 a formed of theCu—X alloy film and the conductive film 159 b formed of the conductivefilm including a low-resistance material.

Alternatively, as illustrated in FIG. 6C, the conductive film 159 havinga three-layer structure is provided between the insulating film 153 andthe oxide semiconductor film 155 b having conductivity. The conductivefilm 159 is formed by stacking the conductive film 159 a formed of theCu—X alloy film, the conductive film 159 b formed of the conductive filmincluding a low-resistance material, and the conductive film 159 cformed of the Cu—X alloy film.

When the conductive film 159 c formed of the Cu—X alloy film is providedover the conductive film 159 b formed of the conductive film including alow-resistance material in the conductive film 159, the conductive film159 c formed of the Cu—X alloy film serves as a protective film of theconductive film 159 b including a low-resistance material; thus, thereaction of the conductive film 159 b including a low-resistancematerial in the formation of the oxide semiconductor film 155 b havingconductivity can be prevented.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, a resistor including the oxide semiconductor filmhaving conductivity in Embodiment 1 is described with reference to FIGS.7A to 7D, FIGS. 8A and 8B, FIGS. 9A and 9B, FIGS. 10A and 10B, and FIGS.11A to 11C.

FIGS. 7A to 7D are cross-sectional views of resistors included in asemiconductor device.

A resistor 160 a in FIG. 7A includes the oxide semiconductor film 155 bhaving conductivity and a pair of conductive films 161 and 162 incontact with the oxide semiconductor film 155 b having conductivity. Theoxide semiconductor film 155 b having conductivity and the pair ofconductive films 161 and 162 are provided over the insulating film 153formed over the substrate 151.

Furthermore, each of the conductive films 161 and 162 may have a singlelayer structure or a stacked-layer structure of two or more layers. Thepair of conductive films 161 and 162 can be formed using a structure, amaterial, and a formation method similar to those of the conductive film159 in Embodiment 1. That is, the pair of conductive films 161 and 162includes the Cu—X alloy film.

In the resistor 160 a in FIG. 7A, the conductive film 161 has astacked-layer structure of a conductive film 161 a in contact with theoxide semiconductor film 155 b having conductivity and a conductive film161 b in contact with the conductive film 161 a, and the conductive film162 has a stacked-layer structure of a conductive film 162 a in contactwith the oxide semiconductor film 155 b having conductivity and aconductive film 162 b in contact with the conductive film 162 a.

Here, as the conductive films 161 a and 162 a, the Cu—X alloy film isused. As the conductive films 161 b and 162 b, the conductive filmincluding a low-resistance material is used.

Furthermore, as in a resistor 160 b illustrated in FIG. 7B, theinsulating film 157 made of a film including hydrogen may be formed overthe insulating film 153, the oxide semiconductor film 155 b havingconductivity, and the pair of conductive films 161 and 162.

Alternatively, as in a resistor 160 c illustrated in FIG. 7C, the oxidesemiconductor film 155 b having conductivity and the pair of conductivefilms 161 and 162 may be formed over the insulating film 157 a made of afilm including hydrogen. In this case, the insulating film 153 a can beprovided over the oxide semiconductor film 155 b having conductivity andthe pair of conductive films 161 and 162.

The resistivity of the oxide semiconductor film 155 b havingconductivity is higher than those of the pair of conductive films 161and 162 including the Cu—X film. Thus, by providing the oxidesemiconductor film 155 b having conductivity between the pair ofconductive films 161 and 162, they serve as a resistor.

The oxide semiconductor film 155 b having conductivity includes defectsand impurities. By the effect of the defects and the impurities, theconductivity of the oxide semiconductor film 155 b having conductivityis increased. Furthermore, the oxide semiconductor film 155 b havingconductivity has a light-transmitting property. As a result, alight-transmitting resistor can be formed.

The pair of conductive films 161 and 162 including the Cu—X alloy filmis formed over the oxide semiconductor film 155 b having conductivity,whereby the adhesion between the oxide semiconductor film 155 b havingconductivity and the pair of conductive films 161 and 162 can beincreased and the contact resistance therebetween can be reduced.

Here, FIG. 7D shows an enlarged view of a region where the oxidesemiconductor film 155 b having conductivity is in contact with theconductive film 161. When the Cu—X alloy film is used as the conductivefilm 161 a in contact with the oxide semiconductor film 155 b havingconductivity, the coating film 156 including X in the Cu—X alloy film isformed at an interface between the oxide semiconductor film 155 b havingconductivity and the conductive film 161 a in some cases. By forming thecoating film 156 serving as a blocking film against Cu, entry of Cu inthe Cu—X alloy film into the oxide semiconductor film 155 b havingconductivity can be suppressed.

Furthermore, although not illustrated, a coating film such as thecoating film 156 a is formed on the periphery of the conductive films161 and 162 in some cases, similarly to the case of the conductive film159 in Embodiment 1.

<Circuit Diagram of Protection Circuit>

A protection circuit using the resistor in this embodiment is describedwith reference to FIGS. 8A and 8B. Although a display device is used asa semiconductor device here, a protection circuit can be used in anothersemiconductor device.

FIG. 8A illustrates a specific example of a protection circuit 170 aincluded in the semiconductor device.

The protection circuit 170 a illustrated in FIG. 8A includes a resistor173 between a wiring 171 and a wiring 172, and a transistor 174 that isdiode-connected.

The resistor 173 is connected to the transistor 174 in series, so thatthe resistor 173 can control the value of current flowing through thetransistor 174 or can function as a protective resistor of thetransistor 174 itself.

The wiring 171 is, for example, a lead wiring from a scan line, a dataline, or a terminal portion included in a display device to a drivercircuit portion. The wiring 172 is, for example, a wiring that issupplied with a potential (VDD, VSS, or GND) of a power supply line forsupplying power to a gate driver or a source driver. Alternatively, thewiring 172 is a wiring that is supplied with a common potential (commonline).

For example, the wiring 172 is preferably connected to the power supplyline for supplying power to a scan line driver circuit, in particular,to a wiring for supplying a low potential. This is because a gate signalline has a low-level potential in most periods, and thus, when thewiring 172 also has a low-level potential, current leaked from the gatesignal line to the wiring 172 can be reduced in a normal operation.

Although the resistor 173 illustrated in FIG. 8A is connected in seriesto the diode-connected transistor, the resistor 173 can be connected inparallel to the diode-connected transistor without being limited to theexample in FIG. 8A.

Next, FIG. 8B illustrates a protection circuit including a plurality oftransistors and a plurality of resistors.

A protection circuit 170 b illustrated in FIG. 8B includes transistors174 a, 174 b, 174 c, and 174 d and resistors 173 a, 173 b, and 173 c.The protection circuit 170 b is provided between a set of wirings 175,176, and 177 and another set of wirings 175, 176, and 177. The wirings175, 176, and 177 are connected to one or more of a scan line drivercircuit, a signal line driver circuit, and a pixel portion. In addition,a first terminal serving as a source electrode of the transistor 174 ais connected to a second terminal serving as a gate electrode of thetransistor 174 a, and a third terminal serving as a drain electrode ofthe transistor 174 a is connected to a wiring 177. A first terminalserving as a source electrode of the transistor 174 b is connected to asecond terminal serving as a gate electrode of the transistor 174 b, anda third terminal serving as a drain electrode of the transistor 174 b isconnected to the first terminal of the transistor 174 a. A firstterminal serving as a source electrode of the transistor 174 c isconnected to a second terminal serving as a gate electrode of thetransistor 174 c, and a third terminal serving as a drain electrode ofthe transistor 174 c is connected to the first terminal of thetransistor 174 b. A first terminal serving as a source electrode of thetransistor 174 d is connected to a second terminal serving as a gateelectrode of the transistor 174 d, and a third terminal serving as adrain electrode of the transistor 174 d is connected to the firstterminal of the transistor 174 c. In addition, the resistors 173 a and173 c are provided in the wiring 177. The resistor 173 b is providedbetween the wiring 176 and the first terminal of the transistor 174 band the third terminal of the transistor 174 c.

Note that the wiring 175 can be used as a power supply line to which thelow power supply potential VSS is applied, for example. The wiring 176can be used as a common line, for example. The wiring 177 can be used asa power supply line to which the high power supply potential VDD isapplied.

The resistor in this embodiment can be used as the resistors in FIGS. 8Aand 8B. By appropriately adjusting the shape, specifically the length orthe width, of the oxide semiconductor film having conductivity includedin the resistor, the resistor can have a given resistance. FIGS. 9A and9B illustrate an example of a resistor 160 d. FIG. 9A is a top view ofthe resistor 160 d, and FIG. 9B is a cross-sectional view taken alongdashed-dotted line A-B in FIG. 9A. As in the resistor 160 d illustratedin FIGS. 9A and 9B, the top surface of an oxide semiconductor film 155 chaving conductivity has a zigzag shape, whereby the resistance of theresistor can be controlled.

In this manner, the protection circuit 170 b includes the plurality oftransistors that are diode-connected and the plurality of resistors. Inother words, the protection circuit 170 b can include diode-connectedtransistors and resistors that are combined in parallel.

With the protection circuit, the semiconductor device can have anenhanced resistance to overcurrent due to electrostatic discharge (ESD).Therefore, a semiconductor device with improved reliability can beprovided.

Furthermore, because the resistor can be used as the protection circuitand the resistance of the resistor can be controlled arbitrarily, thediode-connected transistor or the like that is used as the protectioncircuit can also be protected.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Modification Example 1

As in a resistor 160 e illustrated in FIG. 10A, each of the conductivefilms 161 a and 162 a can be formed of a single layer of the Cu—X alloyfilm over the oxide semiconductor film 155 b having conductivity.

Alternatively, as in a resistor 160 f illustrated in FIG. 10B, the pairof conductive films 161 and 162 can have a three-layer structure. Theconductive film 161 has a stacked-layer structure of the conductive film161 a in contact with the oxide semiconductor film 155 b havingconductivity, the conductive film 161 b in contact with the conductivefilm 161 a, and a conductive film 161 c in contact with the conductivefilm 161 b. The conductive film 162 has a stacked-layer structure of theconductive film 162 a in contact with the oxide semiconductor film 155 bhaving conductivity, the conductive film 162 b in contact with theconductive film 162 a, and a conductive film 162 c in contact with theconductive film 162 b.

When the pair of conductive films 161 and 162 includes the conductivefilms 161 c and 162 c formed of the Cu—X alloy film over the conductivefilms 161 b and 162 b formed of the conductive film including alow-resistance material, the conductive films 161 c and 162 c formed ofthe Cu—X alloy film serve as protective films of the conductive films161 b and 162 b including a low-resistance material; thus, the reactionof the conductive films 161 b and 162 b including a low-resistancematerial in the formation of the insulating film 157 can be prevented.

Furthermore, although not illustrated, a coating film such as thecoating films 156 b and 156 c is formed on the periphery of theconductive films 161 and 162 in some cases, similarly to the case of theconductive film 159 in Embodiment 1.

Modification Example 2

Here, a modification example of a resistor is described with referenceto FIGS. 11A to 11C.

A resistor 160 g in FIG. 11A includes the pair of conductive films 163 aand 164 a formed of the single-layer Cu—X alloy film between theinsulating film 153 and the oxide semiconductor film 155 b havingconductivity.

Alternatively, as illustrated in FIG. 11B, in a resistor 160 h, the pairof conductive films 163 and 164 is provided between the insulating film153 and the oxide semiconductor film 155 b having conductivity and has atwo-layer structure. The conductive film 163 is formed by stacking theconductive film 163 a formed of the Cu—X alloy film and the conductivefilm 163 b formed of the conductive film including a low-resistancematerial. The conductive film 164 is formed by stacking the conductivefilm 164 a formed of the Cu—X alloy film and the conductive film 164 bformed of the conductive film including a low-resistance material.

Alternatively, as illustrated in FIG. 11C, in a resistor 160 i, the pairof conductive films 163 and 164 is provided between the insulating film153 and the oxide semiconductor film 155 b having conductivity and has athree-layer structure. The conductive film 163 is formed by stacking theconductive film 163 a formed of the Cu—X alloy film, the conductive film163 b formed of the conductive film including a low-resistance material,and the conductive film 163 c formed of the Cu—X alloy film. Theconductive film 164 is formed by stacking the conductive film 164 aformed of the Cu—X alloy film, the conductive film 164 b formed of theconductive film including a low-resistance material, and the conductivefilm 164 c formed of the Cu—X alloy film.

When the conductive films 163 c and 164 c formed of the Cu—X alloy filmare provided over the conductive films 163 b and 164 b formed of theconductive film including a low-resistance material in the pair ofconductive films 163 and 164, the conductive films 163 c and 164 cformed of the Cu—X alloy film serve as protective films of theconductive films 163 b and 164 b formed of a conductive film including alow-resistance material; thus, the reaction of the conductive films 163b and 164 b including a low-resistance material in the formation of theoxide semiconductor film 155 b having conductivity and the insulatingfilm 157 can be prevented.

Furthermore, although not illustrated, a coating film such as thecoating films 156, 156 a, 156 b and 156 c is formed on the periphery ofthe pair of conductive films 163 and 164 in some cases, similarly to thecase of the conductive film 159 in Embodiment 1.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, a capacitor including the oxide semiconductor filmhaving conductivity in Embodiment 1 is described with reference to FIGS.12A to 12C, FIGS. 13A and 13B, and FIGS. 14A to 14C.

FIGS. 12A to 12C are cross-sectional views of capacitors included in asemiconductor device.

A capacitor 180 a in FIG. 12A includes the oxide semiconductor film 155b having conductivity, the insulating film 157 in contact with the oxidesemiconductor film 155 b having conductivity, and a conductive film 181overlapping with the oxide semiconductor film 155 b with the insulatingfilm 157 therebetween. Furthermore, a conductive film serving as a leadwiring may be in contact with the oxide semiconductor film 155 b havingconductivity or the conductive film 181. Here, the conductive film 159in contact with the oxide semiconductor film 155 b having conductivityis the film serving as a lead wiring. The oxide semiconductor film 155 bhaving conductivity, the insulating film 157, and the conductive film159 are provided over the insulating film 153 formed over the substrate151.

Furthermore, the conductive film 159 may have a single layer structureor a stacked-layer structure of two or more layers. The conductive film159 can be formed using a structure, a material, and a formation methodsimilar to those of the conductive film 159 in Embodiment 1. That is,the conductive film 159 includes the Cu—X alloy film.

In the capacitor 180 a in FIG. 12A, the conductive film 159 has astacked-layer structure of a conductive film 159 a in contact with theoxide semiconductor film 155 b having conductivity and a conductive film159 b in contact with the conductive film 159 a. As the conductive film159 a, the Cu—X alloy film is used. As the conductive film 159 b, theconductive film including a low-resistance material is used.

Alternatively, as in a capacitor 180 b illustrated in FIG. 12B, theoxide semiconductor film 155 b having conductivity and the conductivefilm 159 may be formed over the insulating film 157 a. In this case, theinsulating film 153 a can be provided between the oxide semiconductorfilm 155 b having conductivity and the conductive film 181.

The conductive film 181 is formed to have a single-layer structure or astacked-layer structure including any of metals such as aluminum,titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum,iron, cobalt, silver, tantalum, and tungsten and an alloy containing anyof these metals as its main component. For example, a single-layerstructure of an aluminum film containing silicon; a single-layerstructure of a copper film containing manganese; a two-layer structurein which an aluminum film is stacked over a titanium film; a two-layerstructure in which an aluminum film is stacked over a tungsten film; atwo-layer structure in which a copper film is stacked over acopper-magnesium-aluminum alloy film; a two-layer structure in which acopper film is stacked over a titanium film; a two-layer structure inwhich a copper film is stacked over a tungsten film; a two-layerstructure in which a copper film is stacked over a copper filmcontaining manganese; a three-layer structure in which a titanium filmor a titanium nitride film, an aluminum film or a copper film, and atitanium film or a titanium nitride film are stacked in this order; athree-layer structure in which a molybdenum film or a molybdenum nitridefilm, an aluminum film or a copper film, and a molybdenum film or amolybdenum nitride film are stacked in this order; a three-layerstructure in which a copper film containing manganese, a copper film,and a copper film containing manganese are stacked in this order; andthe like can be given.

For the conductive film 181, a structure and a material similar to thoseof the conductive film 159 can be used as appropriate.

Furthermore, as the conductive film 181, a light-transmitting conductivefilm can be used. As the light-transmitting conductive film, an indiumoxide film containing tungsten oxide, an indium zinc oxide filmcontaining tungsten oxide, an indium oxide film containing titaniumoxide, an indium tin oxide film containing titanium oxide, an indium tinoxide (hereinafter, referred to as ITO) film, an indium zinc oxide film,an indium tin oxide film to which silicon oxide is added, and the likeare given.

The oxide semiconductor film 155 b having conductivity includes defectsand impurities. By the action of the defects and the impurities, theconductivity of the oxide semiconductor film 155 b having conductivityis increased. Furthermore, the oxide semiconductor film 155 b havingconductivity has a light-transmitting property. A light-transmittingconductive film is used as the conductive film 181, whereby alight-transmitting capacitor can be formed.

The conductive film 159 including the Cu—X alloy film is formed over theoxide semiconductor film 155 b having conductivity, whereby the adhesionbetween the oxide semiconductor film 155 b having conductivity and theconductive film 159 can be increased and the contact resistance betweenthem can be reduced.

Here, FIG. 12C shows an enlarged view of a region where the oxidesemiconductor film 155 b having conductivity is in contact with theconductive film 159. When the Cu—X alloy film is used as the conductivefilm 159 a in contact with the oxide semiconductor film 155 b havingconductivity, the coating film 156 including X in the Cu—X alloy film isformed at an interface between the oxide semiconductor film 155 b havingconductivity and the conductive film 159 a in some cases. By forming thecoating film 156 serving as a blocking film against Cu, entry of Cu inthe Cu—X alloy film into the oxide semiconductor film 155 b havingconductivity can be suppressed.

Furthermore, although not illustrated, a coating film such as thecoating film 156 a is formed on the periphery of the conductive films159 in some cases, similarly to the case of the conductive film 159 inEmbodiment 1.

Modification Example 1

As in a capacitor 180 c illustrated in FIG. 13A, a single layer of theconductive film 159 a formed of the Cu—X alloy film can be formed overthe oxide semiconductor film 155 b having conductivity.

Alternatively, as in a capacitor 180 d illustrated in FIG. 13B, theconductive film 159 can have a three-layer structure. The conductivefilm 159 has a stacked-layer structure of the conductive film 159 a incontact with the oxide semiconductor film 155 b having conductivity, theconductive film 159 b in contact with the conductive film 159 a, and theconductive film 159 c in contact with the conductive film 159 b.

When the conductive film 159 c formed of the Cu—X alloy film is providedover the conductive film 159 b formed of the conductive film including alow-resistance material in the conductive film 159, the conductive film159 c formed of the Cu—X alloy film serves as a protective film of theconductive film 159 b including a low-resistance material; thus, thereaction of the conductive film 159 b including a low-resistancematerial in the formation of the insulating film 157 can be prevented.

Furthermore, although not illustrated, a coating film such as thecoating films 156 b and 156 c is formed on the periphery of theconductive film 159 in some cases, similarly to the case of theconductive film 159 in Embodiment 1.

Modification Example 2

Here, a modification example of a capacitor is described with referenceto FIGS. 14A to 14C.

A capacitor 180 e in FIG. 14A includes the conductive film 159 a formedof the single-layer Cu—X alloy film between the insulating film 153 andthe oxide semiconductor film 155 b having conductivity.

Alternatively, as illustrated in FIG. 14B, in a capacitor 180 f, theconductive film 159 is provided between the insulating film 153 and theoxide semiconductor film 155 b having conductivity and has a two-layerstructure. The conductive film 159 is formed by stacking the conductivefilm 159 a formed of the Cu—X alloy film and the conductive film 159 bformed of the conductive film including a low-resistance material.

Alternatively, as illustrated in FIG. 14C, in a capacitor 180 g, theconductive film 159 is provided between the insulating film 153 and theoxide semiconductor film 155 b having conductivity and has a three-layerstructure. The conductive film 159 is formed by stacking the conductivefilm 159 a formed of the Cu—X alloy film, the conductive film 159 bformed of the conductive film including a low-resistance material, andthe conductive film 159 c formed of the Cu—X alloy film.

When the conductive film 159 c formed of the Cu—X alloy film is providedover the conductive film 159 b formed of the conductive film including alow-resistance material in the conductive film 159, the conductive film159 c formed of the Cu—X alloy film serves as a protective film of theconductive film 159 b including a low-resistance material; thus, thereaction of the conductive film 159 b including a low-resistancematerial in the formation of the oxide semiconductor film 155 b havingconductivity and the insulating film 157 can be prevented.

Furthermore, although not illustrated, a coating film such as thecoating films 156, 156 a, 156 b and 156 c is formed on the periphery ofthe conductive film 159 in some cases, similarly to the case of theconductive film 159 in Embodiment 1.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 4

In this embodiment, a display device of one embodiment of the presentinvention is described with reference to drawings. A semiconductordevice provided with a capacitor including the oxide semiconductor filmhaving conductivity in Embodiment 1 is described with reference to FIGS.15A to 15C, FIG. 16, FIG. 17, FIGS. 18A to 18D, FIGS. 19A to 19C, FIGS.20A to 20C, FIGS. 21A and 21B, FIG. 22, FIG. 23, FIG. 24, FIG. 25, andFIGS. 26A and 26B.

FIG. 15A illustrates an example of a display device. A display deviceillustrated in FIG. 15A includes a pixel portion 101; a scan line drivercircuit 104; a signal line driver circuit 106; m scan lines 107 whichare arranged parallel or substantially parallel to each other and whosepotentials are controlled by the scan line driver circuit 104; and nsignal lines 109 which are arranged parallel or substantially parallelto each other and whose potentials are controlled by the signal linedriver circuit 106. The pixel portion 101 further includes a pluralityof pixels 103 arranged in a matrix. Furthermore, capacitor lines 115arranged parallel or substantially parallel may be provided along thesignal lines 109. Note that the capacitor lines 115 may be arrangedparallel or substantially parallel along the scan lines 107. The scanline driver circuit 104 and the signal line driver circuit 106 arecollectively referred to as a driver circuit portion in some cases.

In addition, the display device also includes a driver circuit fordriving a plurality of pixels, and the like. The display device may alsobe referred to as a liquid crystal module including a control circuit, apower supply circuit, a signal generation circuit, a backlight module,and the like provided over another substrate.

Each scan line 107 is electrically connected to the n pixels 103 in thecorresponding row among the pixels 103 arranged in m rows and n columnsin the pixel portion 101. Each signal line 109 is electrically connectedto the m pixels 103 in the corresponding column among the pixels 103arranged in m rows and n columns. Note that m and n are each an integerof 1 or more. Each capacitor line 115 is electrically connected to the mpixels 103 in the corresponding columns among the pixels 103 arranged inm rows and n columns. Note that in the case where the capacitor lines115 are arranged parallel or substantially parallel along the scan lines107, each capacitor line 115 is electrically connected to the n pixels103 in the corresponding rows among the pixels 103 arranged in m rowsand n columns.

In the case where FFS driving is used for a liquid crystal displaydevice, the capacitor line is not provided and a common line or a commonelectrode serves as a capacitor line.

Note that here, a pixel refers to a region surrounded by scan lines andsignal lines and exhibiting one color. Therefore, in the case of a colordisplay device having color elements of R (red), G (green), and B(blue), a minimum unit of an image is composed of three pixels of an Rpixel, a G pixel, and a B pixel. Note that color reproducibility can beimproved by adding a yellow pixel, a cyan pixel, a magenta pixel, or thelike to the R pixel, the G pixel, and the B pixel. Moreover, powerconsumption of the display device can be reduced by adding a W (white)pixel to the R pixel, the G pixel, and the B pixel. In the case of aliquid crystal display device, brightness of the liquid crystal displaydevice can be improved by adding a W pixel to each of the R pixel, the Gpixel, and the B pixel. As a result, the power consumption of the liquidcrystal display device can be reduced.

FIGS. 15B and 15C illustrate examples of a circuit configuration thatcan be used for the pixels 103 in the display device illustrated in FIG.15A.

The pixel 103 in FIG. 15B includes a liquid crystal element 121, atransistor 102, and a capacitor 105.

The potential of one of a pair of electrodes of the liquid crystalelement 121 is set as appropriate according to the specifications of thepixel 103. The alignment state of the liquid crystal element 121 dependson written data. A common potential may be supplied to one of the pairof electrodes of the liquid crystal element 121 included in each of aplurality of pixels 103. Furthermore, the potential supplied to the oneof the pair of electrodes of the liquid crystal element 121 in the pixel103 in one row may be different from the potential supplied to the oneof the pair of electrodes of the liquid crystal element 121 in the pixel103 in another row.

The liquid crystal element 121 is an element that controls transmissionor non-transmission of light utilizing an optical modulation action ofliquid crystal. Note that the optical modulation action of the liquidcrystal is controlled by an electric field applied to the liquid crystal(including a horizontal electric field, a vertical electric field, and adiagonal electric field). Examples of the liquid crystal element 121 area nematic liquid crystal, a cholesteric liquid crystal, a smectic liquidcrystal, a thermotropic liquid crystal, a lyotropic liquid crystal, aferroelectric liquid crystal, and an anti-ferroelectric liquid crystal.

As examples of a driving method of the display device including theliquid crystal element 121, any of the following modes can be given: aTN mode, a VA mode, an ASM (axially symmetric aligned micro-cell) mode,an OCB (optically compensated birefringence) mode, an MVA mode, a PVA(patterned vertical alignment) mode, an IPS mode, an FFS mode, a TBA(transverse bend alignment) mode, and the like. Note that one embodimentof the present invention is not limited to this, and various liquidcrystal elements and driving methods can be used as a liquid crystalelement and a driving method thereof.

The liquid crystal element may be formed using a liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral material. The liquid crystal exhibiting a blue phase has a shortresponse time of 1 msec or less and is optically isotropic; therefore,alignment treatment is not necessary and viewing angle dependence issmall.

In the structure of the pixel 103 illustrated in FIG. 15B, one of asource electrode and a drain electrode of the transistor 102 iselectrically connected to the signal line 109, and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 121. A gate electrode of the transistor 102 iselectrically connected to the scan line 107. The transistor 102 has afunction of controlling whether to write a data signal by being turnedon or off.

In the pixel 103 in FIG. 15B, one of a pair of electrodes of thecapacitor 105 is electrically connected to the capacitor line 115 towhich a potential is supplied, and the other thereof is electricallyconnected to the other of the pair of electrodes of the liquid crystalelement 121. The potential of the capacitor line 115 is set asappropriate in accordance with the specifications of the pixel 103. Thecapacitor 105 functions as a storage capacitor for storing written data.

The pixel 103 in FIG. 15C includes a transistor 133 performing switchingof a display element, the transistor 102 controlling pixel driving, atransistor 135, the capacitor 105, and a light-emitting element 131.

One of a source electrode and a drain electrode of the transistor 133 iselectrically connected to the signal line 109 to which a data signal issupplied. A gate electrode of the transistor 133 is electricallyconnected to a scan line 107 to which a gate signal is supplied.

The transistor 133 has a function of controlling whether to write a datasignal by being turned on or off.

One of a source electrode and a drain electrode of the transistor 102 iselectrically connected to a wiring 137 serving as an anode line, and theother is electrically connected to one electrode of the light-emittingelement 131. The gate electrode of the transistor 102 is electricallyconnected to the other of the source electrode and the drain electrodeof the transistor 133 and one electrode of the capacitor 105.

The transistor 102 has a function of controlling current flowing throughthe light-emitting element 131 by being turned on or off.

One of a source electrode and a drain electrode of the transistor 135 isconnected to a wiring 139 to which a reference potential of data issupplied, and the other thereof is electrically connected to the oneelectrode of the light-emitting element 131 and the other electrode ofthe capacitor 105. Moreover, a gate electrode of the transistor 135 iselectrically connected to the scan line 107 to which the gate signal issupplied.

The transistor 135 has a function of adjusting the current flowingthrough the light-emitting element 131. For example, when the internalresistance of the light-emitting element 131 increases because ofdeterioration or the like of the light-emitting element 131, the currentflowing through the light-emitting element 131 can be corrected bymonitoring current flowing through the wiring 139 to which the one ofthe source electrode and the drain electrode of the transistor 135 isconnected. The potential supplied to the wiring 139 can be set to 0 V,for example.

The one electrode of the capacitor 105 is electrically connected to thegate electrode of the transistor 102 and the other of the sourceelectrode and the drain electrode of the transistor 133, and the otherelectrode of the capacitor 105 is electrically connected to the other ofthe source electrode and the drain electrode of the transistor 135 andthe one electrode of the light-emitting element 131.

In the pixel 103 in FIG. 15C, the capacitor 105 functions as a storagecapacitor for storing written data.

The one electrode of the light-emitting element 131 is electricallyconnected to the other of the source electrode and the drain electrodeof the transistor 135, the other electrode of the capacitor 105, and theother of the source electrode and the drain electrode of the transistor102. Furthermore, the other electrode of the light-emitting element 131is electrically connected to a wiring 141 serving as a cathode.

As the light-emitting element 131, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 131 is not limited to anorganic EL element; an inorganic EL element including an inorganicmaterial may be used.

A high power supply potential VDD is supplied to one of the wiring 137and the wiring 141, and a low power supply potential VSS is supplied tothe other. In the structure of FIG. 15C, the high power supply potentialVDD is supplied to the wiring 137, and the low power supply potentialVSS is supplied to the wiring 141.

Note that although FIGS. 15B and 15C each illustrate an example wherethe liquid crystal element 121 and the light-emitting element 131 areused as a display element, one embodiment of the present invention isnot limited thereto. Any of a variety of display elements can be used.Examples of a display element include a display medium whose contrast,luminance, reflectance, transmittance, or the like is changed byelectromagnetic action, such as an LED (e.g., a white LED, a red LED, agreen LED, or a blue LED), a transistor (a transistor that emits lightdepending on current), an electron emitter, electronic ink, anelectrophoretic element, a grating light valve (GLV), a plasma displaypanel (PDP), a display element using micro electro mechanical system(MEMS), a digital micromirror device (DMD), a digital micro shutter(DMS), an interferometric modulator display (IMOD) element, a MEMSshutter display element, an optical-interference-type MEMS displayelement, an electrowetting element, a piezoelectric ceramic display, ora carbon nanotube. Note that examples of display devices including ELelements include an EL display. Examples of display devices includingelectron emitters are a field emission display (FED) and an SED-typeflat panel display (SED: surface-conduction electron-emitter display).Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display) and the like. An example of a display device includingelectronic ink or electrophoretic elements is electronic paper. In thecase of a transflective liquid crystal display or a reflective liquidcrystal display, some of or all of pixel electrodes function asreflective electrodes. For example, some or all of pixel electrodes areformed to contain aluminum, silver, or the like. In such a case, amemory circuit such as an SRAM can be provided under the reflectiveelectrodes, leading to lower power consumption.

Next, a specific structure of an element substrate included in thedisplay device is described. Here, a specific example of a liquidcrystal display device including a liquid crystal element in the pixel103 is described. FIG. 16 is a top view of the pixel 103 shown in FIG.15B.

Here, a liquid crystal display device driven in an FFS mode is used asthe display device, and FIG. 16 is a top view of a plurality of pixels103 a, 103 b, and 103 c included in the liquid crystal display device.

In FIG. 16, a conductive film 13 functioning as a scan line extends in adirection substantially perpendicularly to a conductive film functioningas a signal line (in the lateral direction in the drawing). Theconductive film 21 a functioning as a signal line extends in a directionsubstantially perpendicularly to the conductive film functioning as ascan line (in the vertical direction in the drawing). Note that theconductive film 13 functioning as a scan line is electrically connectedto the scan line driver circuit 104 (see FIG. 15A), and the conductivefilm 21 a functioning as a signal line is electrically connected to thesignal line driver circuit 106 (see FIG. 15A).

The transistor 102 is provided in a region where the conductive filmfunctioning as a scan line and the conductive film functioning as asignal line intersect with each other. The transistor 102 includes theconductive film 13 functioning as a gate electrode; a gate insulatingfilm (not illustrated in FIG. 16); the oxide semiconductor film 19 awhere a channel region is formed, over the gate insulating film; and theconductive film 21 a and a conductive film 21 b functioning as a sourceelectrode and a drain electrode. The conductive film 13 also functionsas a conductive film functioning as a scan line, and a region of theconductive film 13 that overlaps with the oxide semiconductor film 19 aserves as the gate electrode of the transistor 102. In addition, theconductive film 21 a also functions as a conductive film functioning asa signal line, and a region of the conductive film 21 a that overlapswith the oxide semiconductor film 19 a functions as the source electrodeor the drain electrode of the transistor 102. Furthermore, in the topview of FIG. 16, an end portion of the conductive film functioning as ascan line is positioned on an outer side of an end portion of the oxidesemiconductor film 19 a. Thus, the conductive film functioning as a scanline functions as a light-blocking film for blocking light from a lightsource such as a backlight. For this reason, the oxide semiconductorfilm 19 a included in the transistor is not irradiated with light, sothat a variation in the electrical characteristics of the transistor canbe suppressed.

In addition, the transistor 102 includes the organic insulating film 31overlapping with the oxide semiconductor film 19 a. The organicinsulating film 31 overlaps with the oxide semiconductor film 19 a (inparticular, a region of the oxide semiconductor film 19 a which isbetween the conductive films 21 a and 21 b) with an inorganic insulatingfilm (not illustrated in FIG. 16) provided therebetween.

Water from the outside does not diffuse to the liquid crystal displaydevice through the organic insulating film 31 because the organicinsulating film 31 is isolated in each transistor 102; therefore, thevariation in electrical characteristics of the transistors provided inthe liquid crystal display device can be reduced.

The conductive film 21 b is electrically connected to an oxidesemiconductor film 19 b having conductivity. A common electrode 29 isprovided over the oxide semiconductor film 19 b having conductivity withan insulating film provided therebetween. An opening 40 indicated by adashed-dotted line is provided in the insulating film provided over theoxide semiconductor film 19 b having conductivity. The oxidesemiconductor film 19 b having conductivity is in contact with a nitrideinsulating film (not illustrated in FIG. 16) in the opening 40.

The common electrode 29 includes stripe regions extending in a directionintersecting with the conductive film 21 a functioning as a signal line.The stripe regions are connected to a region extending in a directionparallel or substantially parallel to the conductive film 21 afunctioning as a signal line. Accordingly, the stripe regions of thecommon electrode 29 are at the same potential in pixels.

The capacitor 105 is formed in a region where the oxide semiconductorfilm 19 b having conductivity and the common electrode 29 overlap witheach other. The oxide semiconductor film 19 b having conductivity andthe common electrode 29 each have a light-transmitting property. Thatis, the capacitor 105 has a light-transmitting property.

As illustrated in FIG. 16, an FFS mode liquid crystal display device isprovided with the common electrode including the stripe regionsextending in a direction intersecting with the conductive filmfunctioning as a signal line. Thus, the display device can haveexcellent contrast.

Owing to the light-transmitting property of the capacitor 105, thecapacitor 105 can be formed large (in a large area) in the pixel 103.Thus, a display device with a large-capacitance capacitor as well as anaperture ratio increased to typically 50% or more, preferably 60% ormore can be provided. For example, in a high-resolution display devicesuch as a liquid crystal display device, the area of a pixel is smalland accordingly the area of a capacitor is also small. For this reason,the amount of charges accumulated in the capacitor is small in thehigh-resolution display device. However, since the capacitor 105 of thisembodiment has a light-transmitting property, when the capacitor 105 isprovided in a pixel, a sufficient capacitance value can be obtained inthe pixel and the aperture ratio can be improved. Typically, thecapacitor 105 can be favorably used for a high-resolution display devicewith a pixel density of 200 pixels per inch (ppi) or more, 300 ppi ormore, or furthermore, 500 ppi or more.

In a liquid crystal display device, as the capacitance value of acapacitor is increased, a period during which the alignment of liquidcrystal molecules of a liquid crystal element can be kept constant inthe state where an electric field is applied can be made longer. Whenthe period can be made longer in a display device which displays a stillimage, the number of times of rewriting image data can be reduced,leading to a reduction in power consumption. Furthermore, according tothe structure of this embodiment, the aperture ratio can be improvedeven in a high-resolution display device, which makes it possible to uselight from a light source such as a backlight efficiently, so that powerconsumption of the display device can be reduced.

Next, FIG. 17 is a cross-sectional view taken along dashed-dotted linesA-B and C-D in FIG. 16. The transistor 102 illustrated in FIG. 17 is achannel-etched transistor. Note that the transistor 102 in the channellength direction and the capacitor 105 are illustrated in thecross-sectional view taken along dashed-dotted line A-B, and thetransistor 102 in the channel width direction is illustrated in thecross-sectional view taken along dashed-dotted line C-D.

The liquid crystal display device described in this embodiment includesa pair of substrates (a first substrate 11 and a second substrate 342),an element layer in contact with the first substrate 11, an elementlayer in contact with the second substrate 342, and a liquid crystallayer 320 provided between the element layers. Note that the elementlayer is a generic term used to refer to layers interposed between thesubstrate and the liquid crystal layer. Furthermore, the substrate andthe element layer are collectively referred to as an element substratein some cases. A liquid crystal element 322 is provided between a pairof substrates (the first substrate 11 and the second substrate 342).

The liquid crystal element 322 includes the oxide semiconductor film 19b having conductivity over the first substrate 11, the common electrode29, a nitride insulating film 27, a film controlling alignment(hereinafter referred to as an alignment film 33), and the liquidcrystal layer 320. The oxide semiconductor film 19 b having conductivityfunctions as one electrode (also referred to as a pixel electrode) ofthe liquid crystal element 322, and the common electrode 29 functions asthe other electrode of the liquid crystal element 322.

First, the element layer formed over the first substrate 11 isdescribed. The transistor 102 in FIG. 17 has a single-gate structure andincludes the conductive film 13 functioning as a gate electrode over thefirst substrate 11. In addition, the transistor 102 includes a nitrideinsulating film 15 formed over the first substrate 11 and the conductivefilm 13 functioning as a gate electrode, an oxide insulating film 17formed over the nitride insulating film 15, the oxide semiconductor film19 a overlapping with the conductive film 13 functioning as a gateelectrode with the nitride insulating film 15 and the oxide insulatingfilm 17 provided therebetween, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode which are incontact with the oxide semiconductor film 19 a. The nitride insulatingfilm 15 and the oxide insulating film 17 function as the gate insulatingfilm 14. Moreover, an oxide insulating film 23 is formed over the oxideinsulating film 17, the oxide semiconductor film 19 a, and theconductive films 21 a and 21 b functioning as a source electrode and adrain electrode, and an oxide insulating film 25 is formed over theoxide insulating film 23. The nitride insulating film 27 is formed overthe oxide insulating film 23, the oxide insulating film 25, and theconductive film 21 b. The oxide insulating film 23, the oxide insulatingfilm 25, and the nitride insulating film 27 function as the inorganicinsulating film 30. The oxide semiconductor film 19 b havingconductivity is formed over the oxide insulating film 17. The oxidesemiconductor film 19 b having conductivity is connected to one of theconductive films 21 a and 21 b functioning as a source electrode and adrain electrode, here, connected to the conductive film 21 b. The commonelectrode 29 is formed over the nitride insulating film 27. In addition,the organic insulating film 31 overlapping with the oxide semiconductorfilm 19 a of the transistor 102 with the inorganic insulating film 30provided therebetween is included.

A structure of the display device is described below in detail.

As the first substrate 11, the substrate 151 described in Embodiment 1can be used as appropriate.

The conductive film 13 functioning as a gate electrode can be formedusing a metal element selected from aluminum, chromium, copper,tantalum, titanium, molybdenum, and tungsten; an alloy containing any ofthese metal elements as a component; an alloy containing any of thesemetal elements in combination; or the like. Furthermore, one or moremetal elements selected from manganese and zirconium may be used. Theconductive film 13 functioning as a gate electrode may have asingle-layer structure or a stacked-layer structure of two or morelayers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which an aluminum film isstacked over a titanium film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film or a tungsten nitride film, a two-layer structure in whicha copper film is stacked over a titanium film, a two-layer structure inwhich a copper film is stacked over a molybdenum film, and a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order can be given. Alternatively, an alloyfilm or a nitride film which contains aluminum and one or more elementsselected from titanium, tantalum, tungsten, molybdenum, chromium,neodymium, and scandium may be used.

For the conductive film 13 serving as a gate electrode, the structureand the material used for the conductive film 159 in Embodiment 1 can beused as appropriate. Alternatively, the light-transmitting conductivefilm described in the description of the conductive film 181 inEmbodiment 3 can be used. Alternatively, the conductive film 13 servingas a gate electrode can have a stacked-layer structure of thelight-transmitting conductive film and the metal element. Alternatively,the conductive film 13 serving as a gate electrode may be formed usingthe oxide semiconductor film 155 b having conductivity in Embodiment 1.

The nitride insulating film 15 can be a nitride insulating film that ishardly permeated by oxygen. Furthermore, a nitride insulating film whichis hardly permeated by oxygen, hydrogen, and water can be used. As thenitride insulating film that is hardly permeated by oxygen and thenitride insulating film that is hardly permeated by oxygen, hydrogen,and water, a silicon nitride film, a silicon nitride oxide film, analuminum nitride film, an aluminum nitride oxide film, or the like isgiven. Instead of the nitride insulating film that is hardly permeatedby oxygen and the nitride insulating film that is hardly permeated byoxygen, hydrogen, and water, an oxide insulating film such as analuminum oxide film, an aluminum oxynitride film, a gallium oxide film,a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitridefilm, a hafnium oxide film, or a hafnium oxynitride film can be used.

The thickness of the nitride insulating film 15 is preferably greaterthan or equal to 5 nm and less than or equal to 100 nm, furtherpreferably greater than or equal to 20 nm and less than or equal to 80nm.

The oxide insulating film 17 may be formed to have a single-layerstructure or a stacked-layer structure using, for example, one or moreof a silicon oxide film, a silicon oxynitride film, a silicon nitrideoxide film, a silicon nitride film, an aluminum oxide film, a hafniumoxide film, a gallium oxide film, and a Ga—Zn-based metal oxide film.

The oxide insulating film 17 may also be formed using a material havinga high relative dielectric constant such as hafnium silicate(HfSiO_(x)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The thickness of the oxide insulating film 17 is preferably greater thanor equal to 5 nm and less than or equal to 400 nm, further preferablygreater than or equal to 10 nm and less than or equal to 300 nm, stillfurther preferably greater than or equal to 50 nm and less than or equalto 250 nm.

The oxide semiconductor film 19 a and the oxide semiconductor film 19 bhaving conductivity are formed at the same time and are formed using ametal oxide film such as an In—Ga oxide film, an In—Zn oxide film, or anIn—M—Zn oxide film (M represents Al, Ga, Y, Zr, Sn, La, Ce, or Nd).Thus, the oxide semiconductor film 19 a and the oxide semiconductor film19 b having conductivity include the same metal element.

However, the number of the defects of the oxide semiconductor film 19 bhaving conductivity is large and the impurity concentration thereof ishigh as compared with the oxide semiconductor film 19 a. Thus, the oxidesemiconductor film 19 b having conductivity has different electricalcharacteristics from the oxide semiconductor film 19 a. Specifically,the oxide semiconductor film 19 a has semiconductor characteristics andthe oxide semiconductor film 19 b having conductivity has conductivity.

The thicknesses of the oxide semiconductor film 19 a and the oxidesemiconductor film 19 b having conductivity are greater than or equal to3 nm and less than or equal to 200 nm, preferably greater than or equalto 3 nm and less than or equal to 100 nm, further preferably greaterthan or equal to 3 nm and less than or equal to 50 nm.

Part of the oxide semiconductor film 19 a serves as the channel regionof the transistor; thus, the energy gap of the oxide semiconductor film19 a is 2 eV or more, preferably 2.5 eV or more, further preferably 3 eVor more. The off-state current of the transistor 102 can be reduced byusing an oxide semiconductor having such a wide energy gap.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 19 a. For example, an oxide semiconductor filmwhose carrier density is 1×10¹⁷/cm³ or lower, preferably 1×10¹⁵/cm³ orlower, preferably 1×10¹³/cm³ or lower, preferably 8×10¹¹/cm³ or lower,preferably 1×10¹¹/cm³ or lower, further preferably lower than1×10¹⁰/cm³, and is 1×10⁻⁹/cm³ or higher is used as the oxidesemiconductor film 19 a.

Note that, without limitation to the compositions described above, amaterial with an appropriate composition may be used depending onrequired semiconductor characteristics and electrical characteristics(e.g., field-effect mobility and threshold voltage) of a transistor.Furthermore, in order to obtain required semiconductor characteristicsof a transistor, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementto oxygen, the interatomic distance, the density, and the like of theoxide semiconductor film 19 a be set to be appropriate.

Note that by using an oxide semiconductor film in which the impurityconcentration is low and density of defect states is low as the oxidesemiconductor film 19 a, in which case a transistor which has moreexcellent electrical characteristics can be manufactured. Here, thestate in which impurity concentration is low and density of defectstates is low (the amount of oxygen vacancies is small) is referred toas “highly purified intrinsic” or “substantially highly purifiedintrinsic”. A highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor has few carrier generation sources, andthus has a low carrier density in some cases. Thus, a transistor inwhich a channel region is formed in the oxide semiconductor film rarelyhas a negative threshold voltage (is rarely normally on). A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has a low density of defect states and accordinglyhas few carrier traps in some cases. Furthermore, the highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorfilm has an extremely low off-state current; even when an element has achannel width of 1×10⁶ μm and a channel length (L) of 10 μm, theoff-state current can be less than or equal to the measurement limit ofa semiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³A, at a voltage (drain voltage) between a source electrode and a drainelectrode of from 1 V to 10 V. Thus, the transistor in which a channelregion is formed in the oxide semiconductor film has a small variationin electrical characteristics and high reliability in some cases. Asexamples of the impurities, hydrogen, nitrogen, alkali metal, andalkaline earth metal are given.

Hydrogen contained in the oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and in addition, an oxygen vacancyis formed in a lattice from which oxygen is released (or a portion fromwhich oxygen is released). Due to entry of hydrogen into the oxygenvacancy, an electron serving as a carrier is generated in some cases.Furthermore, in some cases, bonding of part of hydrogen to oxygen bondedto a metal atom causes generation of an electron serving as a carrier.Thus, a transistor including an oxide semiconductor which containshydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible as well as the oxygen vacancies in the oxide semiconductor film19 a. Specifically, in the oxide semiconductor film 19 a, the hydrogenconcentration which is measured by secondary ion mass spectrometry(SIMS) is set to be lower than or equal to 5×10¹⁹ atoms/cm³, preferablylower than or equal to 1×10¹⁹ atoms/cm³, further preferably lower thanor equal to 5×10¹⁸ atoms/cm³, still further preferably lower than orequal to 1×10¹⁸ atoms/cm³, yet still further preferably lower than orequal to 5×10¹⁷ atoms/cm³, yet still furthermore preferably lower thanor equal to 1×10¹⁶ atoms/cm³.

When silicon or carbon which is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 19 a, oxygen vacancies areincreased, and the oxide semiconductor film 19 a becomes an n-type film.Thus, the concentration of silicon or carbon (the concentration ismeasured by SIMS) of the oxide semiconductor film 19 a is set to belower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equalto 2×10¹⁷ atoms/cm³.

The concentration of alkali metal or alkaline earth metal in the oxidesemiconductor film 19 a, which is measured by SIMS, is set to be lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generatecarriers when bonded to an oxide semiconductor, in which case theoff-state current of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal in the oxide semiconductor film 19 a.

Furthermore, when containing nitrogen, the oxide semiconductor film 19 aeasily has n-type conductivity by generation of electrons serving ascarriers and an increase of carrier density. Thus, a transistorincluding an oxide semiconductor which contains nitrogen is likely to benormally on. For this reason, nitrogen in the oxide semiconductor filmis preferably reduced as much as possible; the nitrogen concentrationwhich is measured by SIMS is preferably set to be, for example, lowerthan or equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 19 b having conductivity is formed byincluding defects, e.g., oxygen vacancies, and impurities in an oxidesemiconductor film formed at the same time as the oxide semiconductorfilm 19 a. Thus, the oxide semiconductor film 19 b having conductivityserves as an electrode, e.g., a pixel electrode in this embodiment.

The oxide semiconductor film 19 a and the oxide semiconductor film 19 bhaving conductivity are both formed over the oxide insulating film 17,but differ in impurity concentration. Specifically, the oxidesemiconductor film 19 b having conductivity has a higher impurityconcentration than the oxide semiconductor film 19 a. For example, thehydrogen concentration in the oxide semiconductor film 19 a is lowerthan or equal to 5×10¹⁹ atoms/cm³, preferably lower than or equal to1×10¹⁹ atoms/cm³, further preferably lower than or equal to 5×10¹⁸atoms/cm³, still further preferably lower than or equal to 1×10¹⁸atoms/cm³, yet further preferably lower than or equal to 5×10¹⁷atoms/cm³, yet furthermore preferably lower than or equal to 1×10¹⁶atoms/cm³. On the other hand, the hydrogen concentration in the oxidesemiconductor film 19 b having conductivity is higher than or equal to8×10¹⁹ atoms/cm³, preferably higher than or equal to 1×10²⁰ atoms/cm³,further preferably higher than or equal to 5×10²⁰ atoms/cm³. Thehydrogen concentration in the oxide semiconductor film 19 b havingconductivity is greater than or equal to 2 times, preferably greaterthan or equal to 10 times that in the oxide semiconductor film 19 a.

The oxide semiconductor film 19 b having conductivity has lowerresistivity than the oxide semiconductor film 19 a. The resistivity ofthe oxide semiconductor film 19 b having conductivity is preferablyhigher than or equal to 1×10⁻⁸ times and lower than 1×10⁻¹ times theresistivity of the oxide semiconductor film 19 a. The resistivity of theoxide semiconductor film 19 b having conductivity is typically higherthan or equal to 1×10⁻³ Ωcm and lower than 1×10⁴Ωcm, preferably higherthan or equal to 1×10⁻³ Ωcm and lower than 1×10⁻¹ Ωcm.

The oxide semiconductor film 19 a and the oxide semiconductor film 19 bhaving conductivity can each have a crystal structure similar to that ofthe oxide semiconductor film 155 b having conductivity in Embodiment 1,as appropriate.

For each of the conductive films 21 a and 21 b serving as a sourceelectrode and a drain electrode, the structure and the material used forthe conductive film 159 in Embodiment 1 can be used as appropriate.

In this embodiment, the conductive film 21 a has a stacked-layerstructure of a conductive film 21 a_1 and a conductive film 21 a_2. Theconductive film 21 b has a stacked-layer structure of a conductive film21 b_1 and a conductive film 21 b_2. As the conductive films 21 a_1 and21 b_1, a Cu—X alloy film is used. As the conductive films 21 a_2 and 21b_2, a conductive film including a low-resistance material is used.

As the oxide insulating film 23 or the oxide insulating film 25, anoxide insulating film which contains more oxygen than that in thestoichiometric composition is preferably used. Here, as the oxideinsulating film 23, an oxide insulating film which permeates oxygen isformed, and as the oxide insulating film 25, an oxide insulating filmwhich contains more oxygen than that in the stoichiometric compositionis formed.

The oxide insulating film 23 is an oxide insulating film through whichoxygen is permeated. Thus, oxygen released from the oxide insulatingfilm 25 provided over the oxide insulating film 23 can be moved to theoxide semiconductor film 19 a through the oxide insulating film 23.Moreover, the oxide insulating film 23 also serves as a film whichrelieves damage to the oxide semiconductor film 19 a at the time offorming the oxide insulating film 25 later.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the oxide insulating film 23.

Furthermore, the oxide insulating film 23 is preferably an oxideinsulating film containing nitrogen and having a small number ofdefects.

Typical examples of the oxide insulating film containing nitrogen andhaving a small number of defects include a silicon oxynitride film andan aluminum oxynitride film.

In an ESR spectrum at 100 K or lower of the oxide insulating film with asmall number of defects, a first signal that appears at a g-factor ofgreater than or equal to 2.037 and less than or equal to 2.039, a secondsignal that appears at a g-factor of greater than or equal to 2.001 andless than or equal to 2.003, and a third signal that appears at ag-factor of greater than or equal to 1.964 and less than or equal to1.966 are observed. The split width of the first and second signals andthe split width of the second and third signals that are obtained by ESRmeasurement using an X-band are each approximately 5 mT. The sum of thespin densities of the first signal that appears at a g-factor of greaterthan or equal to 2.037 and less than or equal to 2.039, the secondsignal that appears at a g-factor of greater than or equal to 2.001 andless than or equal to 2.003, and the third signal that appears at ag-factor of greater than or equal to 1.964 and less than or equal to1.966 is lower than 1×10¹⁸ spins/cm³, typically higher than or equal to1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 correspond to signals attributed to nitrogenoxide (NO_(x); x is greater than or equal to 0 and less than or equal to2, preferably greater than or equal to 1 and less than or equal to 2).Typical examples of nitrogen oxide include nitrogen monoxide andnitrogen dioxide. In other words, the lower the total spin density ofthe first signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is, the lower thecontent of nitrogen oxide in the oxide insulating film is.

When the oxide insulating film 23 contains a small amount of nitrogenoxide as described above, the carrier trap at the interface between theoxide insulating film 23 and the oxide semiconductor film can bereduced. Thus, the amount of change in the threshold voltage of thetransistor included in the semiconductor device can be reduced, whichleads to a reduced change in the electrical characteristics of thetransistor.

The oxide insulating film 23 preferably has a nitrogen concentrationmeasured by secondary ion mass spectrometry (SIMS) of lower than orequal to 6×10²⁰ atoms/cm³. In that case, nitrogen oxide is unlikely tobe generated in the oxide insulating film 23, so that the carrier trapat the interface between the oxide insulating film 23 and the oxidesemiconductor film 19 a can be reduced. Furthermore, the amount ofchange in the threshold voltage of the transistor included in thesemiconductor device can be reduced, which leads to a reduced change inthe electrical characteristics of the transistor.

Note that when nitride oxide and ammonia are contained in the oxideinsulating film 23, the nitride oxide and ammonia react with each otherin heat treatment in the manufacturing process; accordingly, the nitrideoxide is released as a nitrogen gas. Thus, the nitrogen concentration ofthe oxide insulating film 23 and the amount of nitrogen oxide thereincan be reduced. Moreover, the carrier trap at the interface between theoxide insulating film 23 and the oxide semiconductor film 19 a can bereduced. Furthermore, the amount of change in threshold voltage of thetransistor included in the semiconductor device can be reduced, whichleads to a reduced change in the electrical characteristics of thetransistor.

Note that in the oxide insulating film 23, all oxygen entering the oxideinsulating film 23 from the outside does not move to the outside of theoxide insulating film 23 and some oxygen remains in the oxide insulatingfilm 23. Furthermore, movement of oxygen occurs in the oxide insulatingfilm 23 in some cases in such a manner that oxygen enters the oxideinsulating film 23 and oxygen contained in the oxide insulating film 23is moved to the outside of the oxide insulating film 23.

When the oxide insulating film through which oxygen passes is formed asthe oxide insulating film 23, oxygen released from the oxide insulatingfilm 25 provided over the oxide insulating film 23 can be moved to theoxide semiconductor film 19 a through the oxide insulating film 23.

The oxide insulating film 25 is formed in contact with the oxideinsulating film 23. The oxide insulating film 25 is formed using anoxide insulating film which contains oxygen at a higher proportion thanthe stoichiometric composition. Part of oxygen is released by heatingfrom the oxide insulating film which contains oxygen at a higherproportion than the stoichiometric composition. The oxide insulatingfilm which contains oxygen at a higher proportion than thestoichiometric composition is an oxide insulating film of which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the surface temperature ofthe oxide insulating film 25 in the TDS analysis is preferably higherthan or equal to 100° C. and lower than or equal to 700° C., or higherthan or equal to 100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used as the oxide insulating film 25.

It is preferable that the amount of defects in the oxide insulating film25 be small and typically, the spin density of a signal that appears atg=2.001 be lower than 1.5×10¹⁸ spins/cm³, further preferably lower thanor equal to 1×10¹⁸ spins/cm³ by ESR measurement. Note that the oxideinsulating film 25 is provided more apart from the oxide semiconductorfilm 19 a than the oxide insulating film 23 is; thus, the oxideinsulating film 25 may have higher defect density than the oxideinsulating film 23.

Like the nitride insulating film 15, the nitride insulating film 27 canbe a nitride insulating film which is hardly permeated by oxygen.Furthermore, a nitride insulating film which is hardly permeated byoxygen, hydrogen, and water can be used.

The nitride insulating film 27 is formed using a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, or the like with a thickness greater than or equalto 50 nm and less than or equal to 300 nm, preferably greater than orequal to 100 nm and less than or equal to 200 nm.

In the case where the oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition is included in theoxide insulating film 23 or the oxide insulating film 25, part of oxygencontained in the oxide insulating film 23 or the oxide insulating film25 can be moved to the oxide semiconductor film 19 a, so that the amountof oxygen vacancies contained in the oxide semiconductor film 19 a canbe reduced.

The threshold voltage of a transistor using an oxide semiconductor filmwith oxygen vacancies shifts negatively with ease, and such a transistortends to be normally on. This is because charges are generated owing tooxygen vacancies in the oxide semiconductor film and the resistance isthus reduced. The transistor having normally-on characteristic causesvarious problems in that malfunction is likely to be caused when inoperation and that power consumption is increased when not in operation,for example. Furthermore, there is a problem in that the amount ofchange in electrical characteristics, typically in threshold voltage, ofthe transistor is increased by change over time or due to a stress test.

However, in the transistor 102 in this embodiment, the oxide insulatingfilm 23 or the oxide insulating film 25 provided over the oxidesemiconductor film 19 a contains oxygen at a higher proportion than thestoichiometric composition. Furthermore, the oxide semiconductor film 19a, the oxide insulating film 23, and the oxide insulating film 25 aresurrounded by the nitride insulating film 15 and the oxide insulatingfilm 17. As a result, oxygen contained in the oxide insulating film 23or the oxide insulating film 25 is moved to the oxide semiconductor film19 a efficiently, so that the amount of oxygen vacancies in the oxidesemiconductor film 19 a can be reduced. Accordingly, a transistor havingnormally-off characteristic is obtained. Furthermore, the amount ofchange in electrical characteristics, typically in threshold voltage, ofthe transistor over time or due to a stress test can be reduced.

The common electrode 29 is formed using a light-transmitting film,preferably a light-transmitting conductive film. As thelight-transmitting conductive film, an indium oxide film containingtungsten oxide, an indium zinc oxide film containing tungsten oxide, anindium oxide film containing titanium oxide, an indium tin oxide filmcontaining titanium oxide, an ITO film, an indium zinc oxide film, anindium tin oxide film to which silicon oxide is added, and the like aregiven.

The common electrode 29 may be formed using the oxide semiconductor film155 b having conductivity in Embodiment 1.

The extending direction of the conductive film 21 a functioning as asignal line and the extending direction of the common electrode 29intersect with each other. Therefore, differences in directions betweenthe electric field between the conductive film 21 a functioning as asignal line and the common electrode 29 and the electric field betweenthe pixel electrode formed using the oxide semiconductor film 19 bhaving conductivity and the common electrode 29 arise and thedifferences form a large angle. Accordingly, in the case where negativeliquid crystal molecules are used, the alignment state of the liquidcrystal molecules in the vicinity of the conductive film functioning asa signal line and the alignment state of the liquid crystal molecules inthe vicinity of the pixel electrode which is generated by an electricfield between the pixel electrodes provided in adjacent pixels and thecommon electrode are less likely to be affected by each other. Thus, achange in the transmittance of the pixels is suppressed. Accordingly,flickers in an image can be reduced.

In the liquid crystal display device having a low refresh rate,alignment of liquid crystal molecules in the vicinity of the conductivefilm 21 a functioning as a signal line is less likely to affectalignment state of liquid crystal molecules in the vicinity of the pixelelectrode due to the electric field between the pixel electrodes in theadjacent pixels and the common electrode 29 even during the retentionperiod. Thus, the transmittance of the pixels in the retention periodcan be held and flickers can be reduced.

The common electrode 29 includes the stripe regions extending in adirection intersecting with the conductive film 21 a functioning as asignal line. Accordingly, in the vicinity of the oxide semiconductorfilm 19 b having conductivity and the conductive film 21 a, unintendedalignment of liquid crystal molecules can be prevented and thus lightleakage can be suppressed. As a result, a display device with excellentcontrast can be manufactured.

Note that the shape of the common electrode 29 is not limited to thatillustrated in FIG. 16, and may be stripe. In the case of a stripeshape, the extending direction may be parallel to the conductive filmfunctioning as a signal line. The common electrode 29 may have a combshape. Alternatively, the common electrode may be formed over the entiresurface of the first substrate 11. Further alternatively, alight-transmitting conductive film different from the oxidesemiconductor film 19 b having conductivity may be formed over thecommon electrode 29 with an insulating film provided therebetween.

The thickness of the organic insulating film 31 is preferably greaterthan or equal to 500 nm and less than or equal to 10 m. The thickness ofthe organic insulating film 31 in FIG. 17 is smaller than a gap betweenthe inorganic insulating film 30 formed over the first substrate 11 andthe element layer formed on the second substrate 342. Therefore, theliquid crystal layer 320 is provided between the organic insulating film31 and the element layer formed on the second substrate 342. In otherwords, the liquid crystal layer 320 is provided between the alignmentfilm 33 over the organic insulating film 31 and an alignment film 352included in the element layer on the second substrate 342.

Note that although not illustrated, the alignment film 33 over theorganic insulating film 31 and the alignment film 352 included in theelement layer on the second substrate 342 may be in contact with eachother. In this case, the organic insulating film 31 functions as aspacer; therefore, the cell gap of the liquid crystal display device canbe maintained with the organic insulating film 31.

Although the alignment film 33 is provided over the organic insulatingfilm in FIG. 17, one embodiment of the present invention is not limitedthereto. Depending on circumstances or conditions, the organicinsulating film 31 may be provided over the alignment film 33. In thiscase, a rubbing step may be performed after the formation of the organicinsulating film 31 over the alignment film 33 instead of directly afterthe formation of the alignment film 33, for example.

When a negative voltage is applied to the conductive film 13 functioningas a gate electrode, an electric field is generated. The electric fieldis not blocked with the oxide semiconductor film 19 a and affects theinorganic insulating film 30; therefore, the surface of the inorganicinsulating film 30 is weakly positively charged. Moreover, when anegative voltage is applied to the conductive film 13 functioning as agate electrode, positively charged particles contained in the air areadsorbed on the surface of the inorganic insulating film 30 and weakpositive electric charges are generated on the surface of the inorganicinsulating film 30.

The surface of the inorganic insulating film 30 is positively charged,so that an electric field is generated and the electric field affectsthe interface between the oxide semiconductor film 19 a and theinorganic insulating film 30. Thus, the interface between the oxidesemiconductor film 19 a and the inorganic insulating film 30 issubstantially in a state to which a positive bias is applied andtherefore the threshold voltage of the transistor shifts in the negativedirection.

On the other hand, the transistor 102 illustrated in this embodimentincludes the organic insulating film 31 over the inorganic insulatingfilm 30. Since the thickness of the organic insulating film 31 is aslarge as 500 nm or more, the electric field generated by application ofa negative voltage to the conductive film 13 functioning as a gateelectrode does not affect the surface of the organic insulating film 31and the surface of the organic insulating film 31 is not positivelycharged easily. In addition, even when positively charged particle inthe air is adsorbed on the surface of the organic insulating film 31,the electric field of the positively charged particle adsorbed on thesurface of the organic insulating film 31 is less likely to affect theinterface between the oxide semiconductor film 19 a and the inorganicinsulating film 30, because the organic insulating film 31 is thick(greater than or equal to 500 nm). Thus, the interface between the oxidesemiconductor film 19 a and the inorganic insulating film 30 is notsubstantially a state to which a positive bias is applied and thereforethe amount of change in threshold voltage of the transistor is small.

Although water or the like diffuses easily in the organic insulatingfilm 31, water from the outside does not diffuse to a semiconductordevice through the organic insulating film 31 because the organicinsulating film is isolated in each transistor 102. In addition, anitride insulating film is included in the inorganic insulating film 30,whereby water diffused from the outside to the organic insulating film31 can be prevented from diffusing to the oxide semiconductor film 19 a.

The alignment film 33 is formed over the common electrode 29, thenitride insulating film 27, and the organic insulating film 31.

Next, a method for manufacturing the transistor 102 and the capacitor105 in FIG. 17 is described with reference to FIGS. 18A to 18D, FIGS.19A to 19C, FIGS. 20A to 20C, and FIGS. 21A and 21B.

As illustrated in FIG. 18A, a conductive film 12 to be the conductivefilm 13 is formed over the first substrate 11. The conductive film 12 isformed by a sputtering method, a chemical vapor deposition (CVD) methodsuch as a metal organic chemical vapor deposition (MOCVD) method, ametal chemical deposition method, an atomic layer deposition (ALD)method, or a plasma-enhanced chemical vapor deposition (PECVD) method,an evaporation method, a pulsed laser deposition (PLD) method, or thelike. When a metal organic chemical vapor deposition (MOCVD) method, ametal chemical deposition method, or an atomic layer deposition (ALD)method is employed, the conductive film is less damaged by plasma.Furthermore, in the case where the oxide semiconductor film 155 b havingconductivity in Embodiment 1 is used as the conductive film 12, themanufacturing method of the oxide semiconductor film 155 b havingconductivity can be used as appropriate.

Here, a glass substrate is used as the first substrate 11. Furthermore,as the conductive film 12, a 100-nm-thick tungsten film is formed by asputtering method.

Next, a mask is formed over the conductive film 12 by a photolithographyprocess using a first photomask. Then, as illustrated in FIG. 18B, partof the conductive film 12 is etched with the use of the mask to form theconductive film 13 functioning as a gate electrode. After that, the maskis removed.

Note that the conductive film 13 functioning as a gate electrode may beformed by an electrolytic plating method, a printing method, an ink-jetmethod, or the like instead of the above formation method.

Here, the tungsten film is etched by a dry etching method to form theconductive film 13 functioning as a gate electrode.

Next, as illustrated in FIG. 18C, over the conductive film 13functioning as a gate electrode, the nitride insulating film 15 and anoxide insulating film 16 to be the oxide insulating film 17 later areformed. Then, over the oxide insulating film 16, an oxide semiconductorfilm 18 to be the oxide semiconductor film 19 a and the oxidesemiconductor film 19 b having conductivity later is formed.

The nitride insulating film 15 and the oxide insulating film 16 are eachformed by a sputtering method, a chemical vapor deposition (CVD) methodsuch as a metal organic chemical vapor deposition (MOCVD) method, ametal chemical deposition method, an atomic layer deposition (ALD)method, or a plasma-enhanced chemical vapor deposition (PECVD) method,an evaporation method, a pulsed laser deposition (PLD) method, a coatingmethod, a printing method, or the like. When a metal organic chemicalvapor deposition (MOCVD) method, a metal chemical deposition method, oran atomic layer deposition (ALD) method is employed, the nitrideinsulating film 15 and the oxide insulating film 16 are less damaged byplasma. When an atomic layer deposition (ALD) method is employed,coverage of the nitride insulating film 15 and the oxide insulating film16 can be increased.

Here, as the nitride insulating film 15, a 300-nm-thick silicon nitridefilm is formed by a plasma CVD method in which silane, nitrogen, andammonia are used as a source gas.

In the case where a silicon oxide film, a silicon oxynitride film, or asilicon nitride oxide film is formed as the oxide insulating film 16, adeposition gas containing silicon and an oxidizing gas are preferablyused as a source gas. Typical examples of the deposition gas containingsilicon include silane, disilane, trisilane, and silane fluoride. As theoxidizing gas, oxygen, ozone, dinitrogen monoxide, and nitrogen dioxidecan be given as examples.

Moreover, in the case of forming a gallium oxide film as the oxideinsulating film 16, a metal organic chemical vapor deposition (MOCVD)method can be employed.

Here, as the oxide insulating film 16, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane and dinitrogenmonoxide are used as a source gas.

The oxide semiconductor film 18 can be formed by a method that issimilar to that of the oxide semiconductor film 155 described inEmbodiment 1 as appropriate.

Here, a 35-nm-thick In—Ga—Zn oxide film is formed as the oxidesemiconductor film by a sputtering method using an In—Ga—Zn oxide target(In:Ga:Zn=1:1:1).

Then, after a mask is formed over the oxide semiconductor film 18 by aphotolithography process using a second photomask, the oxidesemiconductor film is partly etched using the mask. Thus, the oxidesemiconductor film 19 a and an oxide semiconductor film 19 c which areisolated from each other as illustrated in FIG. 18D are formed. Afterthat, the mask is removed.

Here, the oxide semiconductor films 19 a and 19 c are formed in such amanner that a mask is formed over the oxide semiconductor film 18 andpart of the oxide semiconductor film 18 is etched by a wet etchingmethod.

Next, as illustrated in FIG. 19A, a conductive film 20 to be theconductive films 21 a and 21 b later is formed. Here, the conductivefilm 20 is a stack of a conductive film 20_1 and a conductive film 20_2.As the conductive films 20_1, a Cu—X alloy film is used. As theconductive films 20_2, a conductive film including a low-resistancematerial is used.

The conductive film 20 can be formed by a method similar to that of theconductive film 159 described in Embodiment 1 as appropriate.

Here, a 50-nm-thick Cu—Mn alloy film and a 300-nm-thick copper film aresequentially stacked by a sputtering method.

Next, a mask is formed over the conductive film 20 by a photolithographyprocess using a third photomask. Then, the conductive film 20 is etchedusing the mask, so that the conductive films 21 a and 21 b serving as asource electrode and a drain electrode are formed as illustrated in FIG.19B. After that, the mask is removed. The conductive film 21 a is astack of the conductive film 21 a_1 formed by etching part of theconductive film 20_1 and the conductive film 21 a_2 formed by etchingpart of the conductive film 20_2. The conductive film 21 b is a stack ofthe conductive film 21 b_1 formed by etching part of the conductive film20_1 and the conductive film 21 b_2 formed by etching part of theconductive film 20_2.

Here, a mask is formed over the copper film by a photolithographyprocess. Then, the Cu—Mn film and the copper film are etched with theuse of the mask, so that the conductive films 21 a and 21 b are formed.By using a wet etching method, the Cu—Mn film and the copper film can beetched in one step.

Next, as illustrated in FIG. 19C, an oxide insulating film 22 to be theoxide insulating film 23 later and an oxide insulating film 24 to be theoxide insulating film 25 later are formed over the oxide semiconductorfilms 19 a and 19 c and the conductive films 21 a and 21 b. The oxideinsulating film 22 and the oxide insulating film 24 can each be formedby a method similar to those of the nitride insulating film 15 and theoxide insulating film 16 as appropriate.

Note that after the oxide insulating film 22 is formed, the oxideinsulating film 24 is preferably formed in succession without exposureto the air. After the oxide insulating film 22 is formed, the oxideinsulating film 24 is formed in succession by adjusting at least one ofthe flow rate of a source gas, pressure, a high-frequency power, and asubstrate temperature without exposure to the air, whereby the impurityconcentration attributed to the atmospheric component at the interfacebetween the oxide insulating film 22 and the oxide insulating film 24can be reduced and oxygen in the oxide insulating film 24 can be movedto the oxide semiconductor film 19 a; accordingly, the amount of oxygenvacancies in the oxide semiconductor film 19 a can be reduced.

The oxide insulating film 22 can be formed using an oxide insulatingfilm containing nitrogen and having a small number of defects which isformed by a CVD method under the conditions where the ratio of anoxidizing gas to a deposition gas is higher than 20 times and lower than100 times, preferably higher than or equal to 40 times and lower than orequal to 80 times and the pressure in a treatment chamber is lower than100 Pa, preferably lower than or equal to 50 Pa.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 22. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, and nitrogen dioxide can be given as examples.

With the use of the above conditions, an oxide insulating film whichpermeates oxygen can be formed as the oxide insulating film 22.Furthermore, by providing the oxide insulating film 22, damage to theoxide semiconductor film 19 a can be reduced in the step of forming theoxide insulating film 24.

Here, as the oxide insulating film 22, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 50 sccm and dinitrogen monoxide with a flow rate of 2000 sccm areused as a source gas, the pressure in the treatment chamber is 20 Pa,the substrate temperature is 220° C., and a high-frequency power of 100W is supplied to parallel-plate electrodes with the use of a 27.12 MHzhigh-frequency power source. Under the above conditions, a siliconoxynitride film containing nitrogen and having a small number of defectscan be formed.

As the oxide insulating film 24, a silicon oxide film or a siliconoxynitride film is formed under the following conditions: the substrateplaced in a treatment chamber of a plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., preferably higher than or equalto 200° C. and lower than or equal to 240° C., the pressure is greaterthan or equal to 100 Pa and less than or equal to 250 Pa, preferablygreater than or equal to 100 Pa and less than or equal to 200 Pa withintroduction of a source gas into the treatment chamber, and ahigh-frequency power of greater than or equal to 0.17 W/cm² and lessthan or equal to 0.5 W/cm², preferably greater than or equal to 0.25W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrodeprovided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 24. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, and nitrogen dioxide can be given as examples.

As the film formation conditions of the oxide insulating film 24, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxygen content in the oxide insulating film 24 becomes higher than thatin the stoichiometric composition. On the other hand, in the film formedat a substrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in a later step. Thus, it ispossible to form an oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition and from whichpart of oxygen is released by heating. Furthermore, the oxide insulatingfilm 22 is provided over the oxide semiconductor film 19 a. Accordingly,in the step of forming the oxide insulating film 24, the oxideinsulating film 22 serves as a protective film of the oxidesemiconductor film 19 a. Consequently, the oxide insulating film 24 canbe formed using the high-frequency power having a high power densitywhile damage to the oxide semiconductor film 19 a is reduced.

Here, as the oxide insulating film 24, a 400-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 200 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as the source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and a high-frequency power of 1500W is supplied to the parallel-plate electrodes with the use of a 27.12MHz high-frequency power source. Note that the plasma CVD apparatus is aparallel-plate plasma CVD apparatus in which the electrode area is 6000cm², and the power per unit area (power density) into which the suppliedpower is converted is 0.25 W/cm².

Furthermore, when the conductive films 21 a and 21 b functioning as asource electrode and a drain electrode is formed, the oxidesemiconductor film 19 a is damaged by the etching of the conductivefilm, so that oxygen vacancies are generated on the back channel side ofthe oxide semiconductor film 19 a (the side of the oxide semiconductorfilm 19 a which is opposite to the side facing the conductive film 13functioning as a gate electrode). However, with the use of the oxideinsulating film which contains oxygen at a higher proportion than thestoichiometric composition as the oxide insulating film 24, the oxygenvacancies generated on the back channel side can be repaired by heattreatment. By this, defects contained in the oxide semiconductor film 19a can be reduced, and thus, the reliability of the transistor 102 can beimproved.

Then, a mask is formed over the oxide insulating film 24 by aphotolithography process using a fourth photomask. Next, as illustratedin FIG. 20A, part of the oxide insulating film 22 and part of the oxideinsulating film 24 are etched with the use of the mask to form the oxideinsulating film 23 and the oxide insulating film 25 having the opening40. After that, the mask is removed.

In the process, the oxide insulating films 22 and 24 are preferablyetched by a dry etching method. As a result, the oxide semiconductorfilm 19 c is exposed to plasma in the etching treatment; thus, theamount of oxygen vacancies in the oxide semiconductor film 19 c can beincreased.

Next, heat treatment is performed. The heat treatment is performedtypically at a temperature higher than or equal to 150° C. and lowerthan or equal to 400° C., preferably higher than or equal to 300° C. andlower than or equal to 400° C., further preferably higher than or equalto 320° C. and lower than or equal to 370° C.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature higher than or equal to the strain pointof the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less), or a raregas (argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like.

By the heat treatment, part of oxygen contained in the oxide insulatingfilm 25 can be moved to the oxide semiconductor film 19 a, so that theamount of oxygen vacancies contained in the oxide semiconductor film 19a can be further reduced.

In the case where water, hydrogen, or the like enters the oxideinsulating film 23 and the oxide insulating film 25 and the nitrideinsulating film 26 has a barrier property against water, hydrogen, orthe like, when the nitride insulating film 26 is formed later and heattreatment is performed, water, hydrogen, or the like contained in theoxide insulating film 23 and the oxide insulating film 25 are moved tothe oxide semiconductor film 19 a, so that defects are generated in theoxide semiconductor film 19 a. However, by the heating, water, hydrogen,or the like contained in the oxide insulating film 23 and the oxideinsulating film 25 can be released; thus, variation in electricalcharacteristics of the transistor 102 can be reduced, and a change inthreshold voltage can be suppressed.

Note that when the oxide insulating film 24 is formed over the oxideinsulating film 22 while being heated, oxygen can be moved to the oxidesemiconductor film 19 a to reduce the amount of oxygen vacancies in theoxide semiconductor film 19 a; thus, the heat treatment is notnecessarily performed.

The heat treatment may be performed after the formation of the oxideinsulating films 22 and 24. However, the heat treatment is preferablyperformed after the formation of the oxide insulating films 23 and 25because a film having higher conductivity can be formed in such a mannerthat oxygen is not moved to the oxide semiconductor film 19 c and oxygenis released from the oxide semiconductor film 19 c because of exposureof the oxide semiconductor film 19 c and then oxygen vacancies aregenerated.

Here, the heat treatment is performed at 350° C. in a mixed atmosphereof nitrogen and oxygen for one hour.

Then, as illustrated in FIG. 20B, the nitride insulating film 26 isformed.

The nitride insulating film 26 can be formed by a method similar tothose of the nitride insulating film 15 and the oxide insulating film 16as appropriate. By forming the nitride insulating film 26 by asputtering method, a CVD method, or the like, the oxide semiconductorfilm 19 c is exposed to plasma; thus, the amount of oxygen vacancies inthe oxide semiconductor film 19 c can be increased.

The oxide semiconductor film 19 c has improved conductivity, and becomesthe oxide semiconductor film 19 b having conductivity. When a siliconnitride film is formed by a plasma CVD method as the nitride insulatingfilm 26, hydrogen contained in the silicon nitride film is diffused tothe oxide semiconductor film 19 c; thus, the conductivity of the oxidesemiconductor film can be enhanced. As a manufacturing method of theoxide semiconductor film 19 b having conductivity, the manufacturingmethod of the oxide semiconductor film 155 b having conductivity inEmbodiment 1 can be used.

In the case where a silicon nitride film is formed by a plasma CVDmethod as the nitride insulating film 26, the substrate placed in thetreatment chamber of the plasma CVD apparatus that is vacuum-evacuatedis preferably held at a temperature higher than or equal to 300° C. andlower than or equal to 400° C., further preferably higher than or equalto 320° C. and lower than or equal to 370° C., so that a dense siliconnitride film can be formed.

In the case where a silicon nitride film is formed, a deposition gascontaining silicon, nitrogen, and ammonia are preferably used as asource gas. As the source gas, a small amount of ammonia compared to theamount of nitrogen is used, whereby ammonia is dissociated in the plasmaand activated species are generated. The activated species cleave a bondbetween silicon and hydrogen which are contained in a deposition gascontaining silicon and a triple bond between nitrogen molecules. As aresult, a dense silicon nitride film having few defects, in which bondsbetween silicon and nitrogen are promoted and bonds between silicon andhydrogen is few, can be formed. On the other hand, when the amount ofammonia is larger than the amount of nitrogen in the source gas,cleavage of a deposition gas containing silicon and cleavage of nitrogenare not promoted, so that a sparse silicon nitride film in which bondsbetween silicon and hydrogen remain and defects are increased is formed.Therefore, in a source gas, the flow ratio of the nitrogen to theammonia is set to be preferably greater than or equal to 5 and less thanor equal to 50, further preferably greater than or equal to 10 and lessthan or equal to 50.

Here, in the treatment chamber of a plasma CVD apparatus, a 50-nm-thicksilicon nitride film is formed as the nitride insulating film 26 by aplasma CVD method in which silane with a flow rate of 50 sccm, nitrogenwith a flow rate of 5000 sccm, and ammonia with a flow rate of 100 sccmare used as the source gas, the pressure in the treatment chamber is 100Pa, the substrate temperature is 350° C., and a high-frequency power of1000 W is supplied to parallel-plate electrodes with a high-frequencypower supply of 27.12 MHz. Note that the plasma CVD apparatus is aparallel-plate plasma CVD apparatus in which the electrode area is 6000cm², and the power per unit area (power density) into which the suppliedpower is converted is 1.7×10¹ W/cm².

Next, heat treatment may be performed. The heat treatment is performedtypically at a temperature higher than or equal to 150° C. and lowerthan or equal to 400° C., preferably higher than or equal to 300° C. andlower than or equal to 400° C., further preferably higher than or equalto 320° C. and lower than or equal to 370° C. As a result, the negativeshift of the threshold voltage can be reduced. Moreover, the amount ofchange in the threshold voltage can be reduced.

Next, although not illustrated, a mask is formed by a photolithographyprocess using a fifth photomask. Then, part of each of the nitrideinsulating film 15, the oxide insulating film 16, the oxide insulatingfilm 23, the oxide insulating film 25, and the nitride insulating film26 is etched using the mask to form the nitride insulating film 27 andan opening through which part of a connection terminal formed at thesame time as the conductive film 13 is exposed. Alternatively, part ofeach of the oxide insulating film 23, the oxide insulating film 25, andthe nitride insulating film 26 is etched to form the nitride insulatingfilm 27 and an opening through which part of a connection terminalformed at the same time as the conductive films 21 a and 21 b isexposed.

Next, as illustrated in FIG. 20C, a conductive film 28 to be the commonelectrode 29 later is formed over the nitride insulating film 27.

The conductive film 28 is formed by a sputtering method, a CVD method,an evaporation method, or the like.

Furthermore, in the case where the oxide semiconductor film 155 b havingconductivity in Embodiment 1 is used as the conductive film 28, themanufacturing method of the oxide semiconductor film 155 b havingconductivity can be used as appropriate.

Then, a mask is formed over the conductive film 28 by a photolithographyprocess using a sixth photomask. Next, as illustrated in FIG. 21A, partof the conductive film 28 is etched with the use of the mask to form thecommon electrode 29. Although not illustrated, the common electrode 29is connected to the connection terminal formed at the same time as theconductive film 13 or the connection terminal formed at the same time asthe conductive films 21 a and 21 b. After that, the mask is removed.

Next, as illustrated in FIG. 21B, the organic insulating film 31 isformed over the nitride insulating film 27. An organic insulating filmcan be formed by a coating method, a printing method, or the like asappropriate.

In the case where the organic insulating film is formed by a coatingmethod, a photosensitive composition, with which the upper surfaces ofthe nitride insulating film 27 and the common electrode 29 are coated,is exposed to light and developed by photolithography process using aseventh photomask, and is then subjected to heat treatment. Note that inthe case where the upper surfaces of the nitride insulating film 27 andthe common electrode 29 are coated with a non-photosensitivecomposition, a resist, with which the upper surface of thenon-photosensitive composition is coated, is processed by aphotolithography process using a seventh mask to form a mask, and thenthe non-photosensitive composition is etched using the mask, whereby theorganic insulating film 31 can be formed.

Through the above process, the transistor 102 is manufactured and thecapacitor 105 can be manufactured.

In this embodiment, the conductive film 21 b including the Cu—X alloyfilm is formed over the oxide semiconductor film 19 b havingconductivity, whereby the adhesion between the oxide semiconductor film19 b having conductivity and the conductive film 21 b can be increasedand the contact resistance between them can be reduced.

The element substrate of the display device described in this embodimentincludes an organic insulating film overlapping with a transistor withan inorganic insulating film provided therebetween. Therefore, a displaydevice in which reliability of the transistor can be improved and whosedisplay quality is maintained can be manufactured.

The element substrate of the display device of this embodiment isprovided with a common electrode whose upper surface has a zigzag shapeand which includes stripe regions extending in a direction intersectingwith the conductive film functioning as a signal line. Therefore, thedisplay device can have excellent contrast. In addition, flickers can bereduced in a liquid crystal display device having a low refresh rate.

In the element substrate of the display device of this embodiment, theoxide semiconductor film having conductivity serving as the pixelelectrode is formed at the same time as the oxide semiconductor film ofthe transistor, in which the channel region is formed; therefore, thetransistor 102 and the capacitor 105 can be formed using six photomasks.The oxide semiconductor film having conductivity functions as the one ofelectrodes of the capacitor. The common electrode also functions as theother of electrodes of the capacitor. Thus, a step of forming anotherconductive film is not needed to form the capacitor, and the number ofsteps of manufacturing the display device can be reduced. The capacitorhas a light-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased. Moreover, power consumption of the display device can bereduced.

Next, the element layer formed on the second substrate 342 is described.A film having a colored property (hereinafter referred to as a coloringfilm 346) is formed on the second substrate 342. The coloring film 346functions as a color filter. Furthermore, a light-blocking film 344adjacent to the coloring film 346 is formed on the second substrate 342.The light-blocking film 344 functions as a black matrix. The coloringfilm 346 is not necessarily provided in the case where the liquidcrystal display device is a monochrome display device, for example.

The coloring film 346 is a coloring film that transmits light in aspecific wavelength range. For example, a red (R) film for transmittinglight in a red wavelength range, a green (G) film for transmitting lightin a green wavelength range, a blue (B) film for transmitting light in ablue wavelength range, or the like can be used.

The light-blocking film 344 preferably has a function of blocking lightin a specific wavelength range, and can be a metal film or an organicinsulating film including a black pigment or the like, for example.

An insulating film 348 is formed on the coloring film 346. Theinsulating film 348 functions as a planarization layer or suppressesdiffusion of impurities in the coloring film 346 to the liquid crystalelement side.

A conductive film 350 is formed on the insulating film 348. Theconductive film 350 is formed using a light-transmitting conductivefilm. The potential of the conductive film 350 is preferably the same asthat of the common electrode 29. In other words, a common potential ispreferably applied to the conductive film 350.

When a voltage for driving the liquid crystal molecules is applied tothe conductive film 21 b, an electric field is generated between theconductive film 21 b and the common electrode 29. Liquid crystalmolecules between the conductive film 21 b and the common electrode 29align due to the effect of the electric field and thus a flicker isgenerated.

However, it is possible to suppress an alignment change of liquidcrystal molecules in a direction perpendicular to the substrate due toan electric field between the conductive film 21 b and the commonelectrode 29 in such a manner that the conductive film 350 is providedto face the common electrode 29 through the liquid crystal layer 320 sothat the common electrode 29 and the conductive film 350 have the samepotential. Accordingly, the alignment state of the liquid crystalmolecules in the region is stabilized. Thus, flickers can be reduced.

Note that the alignment film 352 is formed on the conductive film 350.

In addition, the liquid crystal layer 320 is formed between thealignment films 33 and 352. The liquid crystal layer 320 is sealedbetween the first substrate 11 and the second substrate 342 with the useof a sealant (not illustrated). The sealant is preferably in contactwith an inorganic material to prevent entry of moisture and the likefrom the outside.

A spacer may be provided between the alignment films 33 and 352 tomaintain the thickness of the liquid crystal layer 320 (also referred toas a cell gap).

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Modification Example 1

FIG. 22 illustrates a modification example of the display device in FIG.17.

In a display device in FIG. 22, an organic resin film is not formed overthe inorganic insulating film 30, and the alignment film 33 is incontact with the whole of the inorganic insulating film 30. As a result,the number of photomasks for forming the element portion over the firstsubstrate 11 can be reduced, and simplification of the manufacturingprocess of the first substrate 11 provided with the element portion canbe achieved.

Modification Example 2

FIG. 23 illustrates a modification example of the display device in FIG.17.

In a display device in FIG. 23, a continuous organic resin film 31 athat is not divided is formed over the nitride insulating film 27.Furthermore, the common electrode 29 is formed over the organic resinfilm 31 a. The organic resin film 31 a serves as a planarization film;thus, irregularity in alignment of liquid crystal molecules included inthe liquid crystal layer can be reduced.

Modification Example 3

FIG. 24 illustrates a modification example of the display device in FIG.17.

The oxide semiconductor film 19 b having conductivity that serves as apixel electrode in FIG. 24 has a slit. Note that the oxide semiconductorfilm 19 b having conductivity may have a comb-like shape.

Modification Example 4

FIG. 25 illustrates a modification example of the display device in FIG.17.

The common electrode 29 in FIG. 25 overlaps with the conductive film 21b with the nitride insulating film 27 provided therebetween. The commonelectrode 29, the nitride insulating film 27, and the conductive film 21b constitute a capacitor 105 b. By providing the capacitor 105 b, thecapacitance value in the pixel can be increased.

Modification Example 5

FIGS. 26A and 26B illustrate modification examples of the transistor 102in FIG. 17.

A transistor 102 d illustrated in FIG. 26A includes an oxidesemiconductor film 19 g and a pair of conductive films 21 c and 21 d,which are formed with a multi-tone photomask. Note that the conductivefilm 21 c has a stacked-layer structure of a conductive film 21 c_1 anda conductive film 21 c_2. The conductive film 21 d has a stacked-layerstructure of a conductive film 21 d_1 and a conductive film 21 d_2. Asthe conductive films 21 c_1 and 21 d_1, a Cu—X alloy film is used. Asthe conductive films 21 c_2 and 21 d_2, a conductive film including alow-resistance material is used.

With the use of a multi-tone photomask, a resist mask having a pluralityof thicknesses can be formed. After the oxide semiconductor film 19 g isformed with the resist mask, the resist mask is exposed to oxygen plasmaor the like and is partly removed; accordingly, a resist mask forforming a pair of conductive films is formed. Therefore, the number ofsteps in the photolithography process in the process of forming theoxide semiconductor film 19 g and the pair of conductive films 21 c and21 d can be reduced.

Note that outside the pair of conductive films 21 c and 21 d, the oxidesemiconductor film 19 g formed with a multi-tone photomask is partlyexposed when seen from the above.

A transistor 102 e illustrated in FIG. 26B is a channel-protectivetransistor.

The transistor 102 e illustrated in FIG. 26B includes the conductivefilm 13 functioning as a gate electrode provided over the firstsubstrate 11, the gate insulating film 14 formed over the firstsubstrate 11 and the conductive film 13 functioning as a gate electrode,the oxide semiconductor film 19 a overlapping with the conductive film13 functioning as a gate electrode with the gate insulating film 14provided therebetween, an inorganic insulating film 30 a covering achannel region and side surfaces of the oxide semiconductor film 19 a,and conductive films 21 e and 21 f functioning as a source electrode anda drain electrode in contact with the oxide semiconductor film 19 a inan opening of the inorganic insulating film 30 a. Note that theconductive film 21 e has a stacked-layer structure of a conductive film21 e_1 and a conductive film 21 e_2. The conductive film 21 f has astacked-layer structure of a conductive film 21 f_1 and a conductivefilm 21 f_2. As the conductive films 21 e_1 and 21 f_1, a Cu—X alloyfilm is used. As the conductive films 21 e_2 and 21 f_2, a conductivefilm including a low-resistance material is used.

In the channel-protective transistor, the oxide semiconductor film 19 ais not damaged by etching for forming the conductive films 21 e and 21 fbecause the oxide semiconductor film 19 a is covered with the inorganicinsulating film 30 a. Therefore, defects in the oxide semiconductor film19 a can be reduced.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 5

In this embodiment, as an example of a display device, a liquid crystaldisplay device driven in a vertical alignment (VA) mode will bedescribed. First, a top view of a plurality of pixels 103 included inthe liquid crystal display device is shown in FIG. 27.

In FIG. 27, a conductive film 13 functioning as a scan line extends in adirection substantially perpendicularly to a conductive film functioningas a signal line (in the lateral direction in the drawing). Theconductive film 21 a functioning as a signal line extends in a directionsubstantially perpendicularly to the conductive film functioning as ascan line (in the longitudinal direction in the drawing). A conductivefilm 21 g functioning as a capacitor line extends in a directionparallel to the signal line. Note that the conductive film 13functioning as a scan line is electrically connected to the scan linedriver circuit 104 (see FIG. 15A), and the conductive film 21 afunctioning as a signal line and the conductive film 21 g functioning asa capacitor line is electrically connected to the signal line drivercircuit 106 (see FIG. 15A).

The transistor 102 is provided in a region where the conductive filmfunctioning as a scan line and the conductive film functioning as asignal line intersect with each other. The transistor 102 includes theconductive film 13 functioning as a gate electrode; a gate insulatingfilm (not illustrated in FIG. 27); the oxide semiconductor film 19 awhere a channel region is formed, over the gate insulating film; and theconductive films 21 a and 21 b functioning as a pair of electrodes. Theconductive film 13 also functions as a scan line, and a region of theconductive film 13 that overlaps with the oxide semiconductor film 19 afunctions as the gate electrode of the transistor 102. In addition, theconductive film 21 a also functions as a signal line, and a region ofthe conductive film 21 a that overlaps with the oxide semiconductor film19 a functions as the source electrode or the drain electrode of thetransistor 102. Furthermore, in the top view of FIG. 27, an end portionof the conductive film functioning as a scan line is positioned on anouter side of an end portion of the oxide semiconductor film 19 a. Thus,the conductive film functioning as a scan line functions as alight-blocking film for blocking light from a light source such as abacklight. For this reason, the oxide semiconductor film 19 a includedin the transistor is not irradiated with light, so that a variation inthe electrical characteristics of the transistor can be suppressed.

In addition, the transistor 102 includes the organic insulating film 31overlapping with the oxide semiconductor film 19 a in a manner similarto that in Embodiment 4. The organic insulating film 31 overlaps withthe oxide semiconductor film 19 a (in particular, a region of the oxidesemiconductor film 19 a which is between the conductive films 21 a and21 b) with an inorganic insulating film (not illustrated in FIG. 27)provided therebetween.

The conductive film 21 b is electrically connected to alight-transmitting conductive film 29 c that functions as a pixelelectrode in an opening 41.

The capacitor 105 is connected to the conductive film 21 g functioningas a capacitor line. The capacitor 105 includes an oxide semiconductorfilm 19 d having conductivity formed over the gate insulating film, adielectric film formed over the transistor 102, and thelight-transmitting conductive film 29 c functioning as a pixelelectrode. The oxide semiconductor film 19 d having conductivity formedover the gate insulating film has a light-transmitting property. Thatis, the capacitor 105 has a light-transmitting property.

Owing to the light-transmitting property of the capacitor 105, thecapacitor 105 can be formed large (in a large area) in the pixel 103.Thus, a display device with a large-capacitance capacitor as well as anaperture ratio increased to typically 55% or more, preferably 60% ormore can be provided. For example, in a high-resolution display devicesuch as a liquid crystal display device, the area of a pixel is smalland accordingly the area of a capacitor is also small. For this reason,the amount of charges accumulated in the capacitor is small in thehigh-resolution display device. However, since the capacitor 105 of thisembodiment has a light-transmitting property, when the capacitor 105 isprovided in a pixel, a sufficient capacitance value can be obtained inthe pixel and the aperture ratio can be improved. Typically, thecapacitor 105 can be favorably used for a high-resolution display devicewith a pixel density of 200 pixels per inch (ppi) or more, 300 ppi ormore, or furthermore, 500 ppi or more.

Furthermore, according to one embodiment of the present invention, theaperture ratio can be improved even in a high-resolution display device,which makes it possible to use light from a light source such as abacklight efficiently, so that power consumption of the display devicecan be reduced.

Next, FIG. 28 is a cross-sectional view taken along dashed-dotted linesA-B and C-D in FIG. 27. The transistor 102 illustrated in FIG. 27 is achannel-etched transistor. Note that the transistor 102 in the channellength direction, a connection portion between the transistor 102 andthe light-transmitting conductive film 29 c functioning as a pixelelectrode, and the capacitor 105 are illustrated in the cross-sectionalview taken along dashed-dotted line A-B, and the transistor 102 in thechannel width direction is illustrated in the cross-sectional view takenalong dashed-dotted line C-D.

Since the liquid crystal display device described in this embodiment isdriven in a VA mode, a liquid crystal element 322 includes thelight-transmitting conductive film 29 c functioning as a pixel electrodeincluded in the element layer of the first substrate 11, the conductivefilm 350 included in the element layer of the second substrate 342, andthe liquid crystal layer 320.

In addition, the transistor 102 in FIG. 28 has a structure similar tothat of the transistor 102 in Embodiment 4. The light-transmittingconductive film 29 c functioning as a pixel electrode connected to oneof the conductive films 21 a and 21 b functioning as a source electrodeand a drain electrode (here, connected to the conductive film 21 b) isformed over the nitride insulating film 27. In the opening 41 of thenitride insulating film 27, the conductive film 21 b is connected to thelight-transmitting conductive film 29 c functioning as a pixelelectrode.

The light-transmitting conductive film 29 c functioning as a pixelelectrode can be formed using as appropriate a material and amanufacturing method similar to those of the common electrode 29 inEmbodiment 4.

The capacitor 105 in FIG. 28 includes the oxide semiconductor film 19 dhaving conductivity formed over the oxide insulating film 17, thenitride insulating film 27, and the light-transmitting conductive film29 c functioning as a pixel electrode.

Over the transistor 102 in this embodiment, the oxide insulating films23 and 25 which are isolated from each other are formed. The oxideinsulating films 23 and 25 which are isolated from each other overlapwith the oxide semiconductor film 19 a.

In addition, the organic insulating film 31 overlapping with the oxidesemiconductor film 19 a is provided over the nitride insulating film 27.The organic insulating film 31 overlapping with the oxide semiconductorfilm 19 a is provided over the transistor 102, whereby the surface ofthe oxide semiconductor film 19 a can be made apart from the surface ofthe organic insulating film 31. Thus, the surface of the oxidesemiconductor film 19 a is not affected by the electric field ofpositively charged particles adsorbed on the surface of the organicinsulating film 31 and therefore the reliability of the transistor 102can be improved.

In the capacitor 105, the oxide semiconductor film 19 d havingconductivity is different from that in Embodiment 4 and is not connectedto the conductive film 21 b. In contrast, the oxide semiconductor film19 d having conductivity is in contact with a conductive film 21 d. Theconductive film 21 d serves as a capacitor line. The oxide semiconductorfilm 19 d having conductivity can be formed in a manner similar to thatof the oxide semiconductor film 19 b having conductivity in Embodiment4. That is, the oxide semiconductor film 19 d having conductivity is ametal oxide film containing the same metal element as the oxidesemiconductor film 19 a.

Next, a method for manufacturing the transistor 102 and the capacitor105 in FIG. 28 is described with reference to FIGS. 29A to 29C and FIGS.30A to 30C.

A conductive film is formed over the first substrate 11 and then etchedusing a mask formed through the first photolithography process inEmbodiment 4, whereby the conductive film 13 functioning as a gateelectrode is formed over the first substrate 11 (see FIG. 29A).

Next, the nitride insulating film 15 and the oxide insulating film 16are formed over the first substrate 11 and the conductive film 13functioning as a gate electrode. Next, an oxide semiconductor film isformed over the oxide insulating film 16 and then etched using a maskformed through the second photolithography process in Embodiment 4,whereby the oxide semiconductor films 19 a and 19 c are formed (see FIG.29B).

Next, a conductive film is formed over the oxide insulating film 16 andthe oxide semiconductor films 19 a and 19 c and then etched using a maskformed through the third photolithography process in Embodiment 4,whereby the conductive films 21 a, 21 b, and 21 d are formed (see FIG.29C). At this time, the conductive film 21 b is formed so as not to bein contact with the oxide semiconductor film 19 c. The conductive film21 d is formed so as to be in contact with the oxide semiconductor film19 c. In the conductive film 21 d, as in the conductive films 21 a and21 b, the conductive film 21 d_1 and the conductive film 21 d_2 arestacked.

Next, an oxide insulating film is formed over the oxide insulating film16, the oxide semiconductor films 19 a and 19 c, and the conductivefilms 21 a, 21 b, and 21 d and then etched using a mask formed throughthe fourth photolithography process in Embodiment 4, whereby the oxideinsulating films 23 and 25 having the opening 40 are formed (see FIG.30A).

Next, a nitride insulating film is formed over the oxide insulating film17, the oxide semiconductor films 19 a and 19 c, the conductive films 21a, 21 b, and 21 d, and the oxide insulating films 23 and 25 and thenetched using a mask formed through the fifth photolithography process inEmbodiment 4, whereby the nitride insulating film 27 having the opening41 through which part of the conductive film 21 b is exposed is formed(see FIG. 30B).

Through the above steps, the oxide semiconductor film 19 c becomes theoxide semiconductor film 19 d having conductivity. When a siliconnitride film is formed later by a plasma CVD method as the nitrideinsulating film 27, hydrogen contained in the silicon nitride film isdiffused to the oxide semiconductor film 19 c; thus, the conductivity ofthe oxide semiconductor film 19 d having conductivity can be enhanced.

Next, a conductive film is formed over the conductive film 21 b and thenitride insulating film 27 and then etched using a mask formed throughthe sixth photolithography process in Embodiment 4, whereby theconductive film 29 c connected to the conductive film 21 b is formed(see FIG. 30C).

From the above, as for a semiconductor device including an oxidesemiconductor film, a semiconductor device with improved electricalcharacteristics can be obtained.

On an element substrate of the semiconductor device described in thisembodiment, one electrode of the capacitor is formed at the same time asthe oxide semiconductor film of the transistor. In addition, thelight-transmitting conductive film functioning as a pixel electrode isused as the other electrode of the capacitor. Thus, a step of forminganother conductive film is not needed to form the capacitor, and thenumber of steps of manufacturing the display device can be reduced.Furthermore, since the pair of electrodes has a light-transmittingproperty, the capacitor has a light-transmitting property. As a result,the area occupied by the capacitor can be increased and the apertureratio in a pixel can be increased.

Modification Example 1

In this embodiment, a display device that can be manufactured with asmall number of masks as compared with that of the semiconductor devicedescribed in Embodiment 4 is described with reference to FIG. 31.

In the display device illustrated in FIG. 31, the number of masks can bereduced by not etching the oxide insulating film 22 and the oxideinsulating film 24 formed over the transistor 102. In addition, thenitride insulating film 27 is formed over the oxide insulating film 24,and an opening 41 a through which part of the conductive film 21 b isexposed is formed in the oxide insulating films 22 and 24 and thenitride insulating film 27. A light-transmitting conductive film 29 dfunctioning as a pixel electrode, which is connected to the conductivefilm 21 b in the opening 41 a, is formed over the nitride insulatingfilm 27.

The conductive film 21 d is formed over the oxide insulating film 17.Since the conductive film 21 d is formed at the same time as theconductive films 21 a and 21 b are formed, an additional photomask isnot needed to form the conductive film 21 d. The conductive film 21 dfunctions as a capacitor line. That is, a capacitor 105 a includes theconductive film 21 d, the oxide insulating film 22, the oxide insulatingfilm 24, the nitride insulating film 27, and the light-transmittingconductive film 29 d functioning as a pixel electrode.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 6

In this embodiment, a display device which is different from the displaydevices in Embodiment 4 and a manufacturing method thereof are describedwith reference to drawings. This embodiment is different from Embodiment4 in that the transistor has a structure in which an oxide semiconductorfilm is provided between different gate electrodes, that is, a dual-gatestructure. Note that the structures similar to those in Embodiment 4 arenot described repeatedly here.

A specific structure of an element layer formed over the first substrate11 included in the display device is described. The transistor providedin the display device of this embodiment is different from that inEmbodiment 4 in that a conductive film 29 b functioning as a gateelectrode and overlapping part of or the whole of each of the conductivefilm 13 functioning as a gate electrode, the oxide semiconductor film 19a, the conductive films 21 a and 21 b, and the oxide insulating film 25is provided. The conductive film 29 b functioning as a gate electrode isconnected to the conductive film 13 functioning as a gate electrode inthe opening 41 a.

A transistor 102 a illustrated in FIG. 32 is a channel-etchedtransistor. Note that the transistor 102 a in the channel lengthdirection and the capacitor 105 a are illustrated in a cross-sectionalview in a portion A-B, and the transistor 102 a in the channel widthdirection and a connection portion between the conductive film 13functioning as a gate electrode and the conductive film 29 b functioningas a gate electrode are illustrated in a cross-sectional view in aportion C-D.

The transistor 102 a in FIG. 32 has a dual-gate structure and includesthe conductive film 13 functioning as a gate electrode over the firstsubstrate 11. In addition, the transistor 102 a includes the nitrideinsulating film 15 formed over the first substrate 11 and the conductivefilm 13 functioning as a gate electrode, the oxide insulating film 17formed over the nitride insulating film 15, the oxide semiconductor film19 a overlapping with the conductive film 13 functioning as a gateelectrode with the nitride insulating film 15 and the oxide insulatingfilm 17 provided therebetween, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode which are incontact with the oxide semiconductor film 19 a. Moreover, the oxideinsulating film 23 is formed over the oxide insulating film 17, theoxide semiconductor film 19 a, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode, and the oxideinsulating film 25 is formed over the oxide insulating film 23. Thenitride insulating film 27 is formed over the nitride insulating film15, the oxide insulating film 23, the oxide insulating film 25, and theconductive film 21 b. The oxide semiconductor film 19 b havingconductivity is formed over the oxide insulating film 17. The oxidesemiconductor film 19 b having conductivity is connected to one of theconductive films 21 a and 21 b functioning as a source electrode and adrain electrode, here, connected to the conductive film 21 b. The commonelectrode 29 and the conductive film 29 b functioning as a gateelectrode are formed over the nitride insulating film 27.

As illustrated in the cross-sectional view in a portion C-D, theconductive film 29 b functioning as a gate electrode is connected to theconductive film 13 functioning as a gate electrode in the opening 41 aprovided in the nitride insulating film 15 and the nitride insulatingfilm 27. That is, the conductive film 13 functioning as a gate electrodeand the conductive film 29 b functioning as a gate electrode have thesame potential.

Thus, by applying voltage at the same potential to each gate electrodeof the transistor 102 a, variation in the initial characteristics can bereduced, and degradation of the transistor 102 a after the—GBT stresstest and a change in the rising voltage of on-state current at differentdrain voltages can be suppressed. In addition, a region where carriersflow in the oxide semiconductor film 19 a becomes larger in the filmthickness direction, so that the amount of carrier movement isincreased. As a result, the on-state current of the transistor 102 a isincreased, and the field-effect mobility is increased. Typically, thefield-effect mobility is greater than or equal to 20 cm²/V·s.

Over the transistor 102 a in this embodiment, the oxide insulating films23 and 25 are formed. The oxide insulating films 23 and 25 overlap withthe oxide semiconductor film 19 a. In the cross-sectional view in thechannel width direction, end portions of the oxide insulating films 23and 25 are positioned on an outer side of an end portion of the oxidesemiconductor film 19 a. Furthermore, in the channel width direction inFIG. 32, the conductive film 29 b functioning as a gate electrode ispositioned at end portions of the oxide insulating films 23 and 25.

An end portion processed by etching or the like of the oxidesemiconductor film is damaged by processing, to produce defects and alsocontaminated by the attachment of an impurity, or the like. Thus, theend portion of the oxide semiconductor film is easily activated byapplication of a stress such as an electric field, thereby easilybecoming n-type (having a low resistance). Therefore, the end portion ofthe oxide semiconductor film 19 a overlapping with the conductive film13 functioning as a gate electrode easily becomes n-type. When the endportion which becomes n-type is provided between the conductive films 21a and 21 b functioning as a source electrode and a drain electrode, theregion which becomes n-type functions as a carrier path, resulting in aparasitic channel. However, as illustrated in the cross-sectional viewin a portion C-D, when the conductive film 29 b functioning as a gateelectrode faces a side surface of the oxide semiconductor film 19 a withthe oxide insulating films 23 and 25 provided therebetween in thechannel width direction, due to the electric field of the conductivefilm 29 b functioning as a gate electrode, generation of a parasiticchannel on the side surface of the oxide semiconductor film 19 a or in aregion including the side surface and the vicinity of the side surfaceis suppressed. As a result, a transistor which has excellent electricalcharacteristics such as a sharp increase in the drain current at thethreshold voltage is obtained.

On an element substrate of the display device described in thisembodiment, the oxide semiconductor film having conductivity functioningas the pixel electrode is formed at the same time as the oxidesemiconductor film of the transistor. The oxide semiconductor filmhaving conductivity also functions as one of electrodes of thecapacitor. The common electrode also functions as the other ofelectrodes of the capacitor. Thus, a step of forming another conductivefilm is not needed to form the capacitor, and the number of steps ofmanufacturing the semiconductor device can be reduced. The capacitor hasa light-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased.

Details of the transistor 102 a are described below. Note that thecomponents with the same reference numerals as those in Embodiment 4 arenot described here.

The conductive film 29 b functioning as a gate electrode can be formedusing a material similar to that of the common electrode 29 inEmbodiment 4.

Next, a method for manufacturing the transistor 102 a and the capacitor105 a in FIG. 32 is described with reference to FIGS. 18A to 18D, FIGS.19A to 19C, FIGS. 20A and 20B, and FIGS. 33A to 33C.

As in Embodiment 4, through the steps illustrated in FIGS. 18A to 20B,the conductive film 13 functioning as a gate electrode, the nitrideinsulating film 15, the oxide insulating film 16, the oxidesemiconductor film 19 a, the oxide semiconductor film 19 b havingconductivity, the conductive films 21 a and 21 b functioning as a sourceelectrode and a drain electrode, the oxide insulating film 22, the oxideinsulating film 24, and the nitride insulating film 26 are formed overthe first substrate 11. In these steps, photolithography processes usingthe first photomask to the fourth photomask are performed.

Next, a mask is formed over the nitride insulating film 26 through aphotolithography process using a fifth photomask, and then part of thenitride insulating film 26 is etched using the mask; thus, the nitrideinsulating film 27 having the opening 41 a is formed as illustrated inFIG. 33A.

Next, as illustrated in FIG. 33B, the conductive film 28 to be thecommon electrode 29 and the conductive film 29 b functioning as a gateelectrode is formed over the conductive film 13 functioning as a gateelectrode, and the nitride insulating film 27.

Then, a mask is formed over the conductive film 28 by a photolithographyprocess using a sixth photomask. Next, as illustrated in FIG. 33C, partof the conductive film 28 is etched with the use of the mask to form thecommon electrode 29 and the conductive film 29 b functioning as a gateelectrode. After that, the mask is removed.

Through the above process, the transistor 102 a is manufactured and thecapacitor 105 a can also be manufactured.

In the transistor described in this embodiment, when the conductive film29 b functioning as a gate electrode faces a side surface of the oxidesemiconductor film 19 a with the oxide insulating films 23 and 25provided therebetween in the channel width direction, due to theelectric field of the conductive film 29 b functioning as a gateelectrode, generation of a parasitic channel on the side surface of theoxide semiconductor film 19 a or in a region including the side surfaceand the vicinity of the side surface is suppressed. As a result, atransistor which has excellent electrical characteristics such as asharp increase in the drain current at the threshold voltage isobtained.

The element substrate of the display device of this embodiment isprovided with a common electrode including a stripe region extending ina direction intersecting with a signal line. Therefore, the displaydevice can have excellent contrast.

On an element substrate of the display device described in thisembodiment, the oxide semiconductor film having conductivity functioningas the pixel electrode is formed at the same time as the oxidesemiconductor film of the transistor. The oxide semiconductor filmhaving conductivity functions as the one of electrodes of the capacitor.The common electrode also functions as the other of electrodes of thecapacitor. Thus, a step of forming another conductive film is not neededto form the capacitor, and the number of steps of manufacturing thedisplay device can be reduced. The capacitor has a light-transmittingproperty. As a result, the area occupied by the capacitor can beincreased and the aperture ratio in a pixel can be increased.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 7

In this embodiment, a display device including a transistor in which thenumber of defects in an oxide semiconductor film can be further reducedas compared with the above embodiments is described with reference todrawings. The transistor described in this embodiment is different fromany of the transistors in Embodiments 4 to 6 in that a multilayer filmincluding a plurality of oxide semiconductor films is provided. Here,details are described using the transistor in Embodiment 4.

FIGS. 34A and 34B each show a cross-sectional view of an elementsubstrate included in a display device. FIGS. 34A and 34B arecross-sectional views taken along dashed-dotted lines A-B and C-D inFIG. 16.

A transistor 102 b in FIG. 34A includes a multilayer film 37 aoverlapping with the conductive film 13 functioning as a gate electrodewith the nitride insulating film 15 and the oxide insulating film 17provided therebetween, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode in contact withthe multilayer film 37 a. The oxide insulating film 23, the oxideinsulating film 25, and the nitride insulating film 27 are formed overthe nitride insulating film 15, the oxide insulating film 17, themultilayer film 37 a, and the conductive films 21 a and 21 b functioningas a source electrode and a drain electrode.

The capacitor 105 b in FIG. 34A includes a multilayer film 37 b formedover the oxide insulating film 17, the nitride insulating film 27 incontact with the multilayer film 37 b, and the common electrode 29 incontact with the nitride insulating film 27. The multilayer film 37 bfunctions as a pixel electrode.

In the transistor 102 b described in this embodiment, the multilayerfilm 37 a includes the oxide semiconductor film 19 a and an oxidesemiconductor film 39 a. That is, the multilayer film 37 a has atwo-layer structure. In addition, part of the oxide semiconductor film19 a functions as a channel region. Moreover, the oxide insulating film23 is formed in contact with the multilayer film 37 a, and the oxideinsulating film 25 is formed in contact with the oxide insulating film23. That is, the oxide semiconductor film 39 a is provided between theoxide semiconductor film 19 a and the oxide insulating film 23.

The oxide semiconductor film 39 a is an oxide film containing one ormore elements that constitute the oxide semiconductor film 19 a. Thus,interface scattering is unlikely to occur at the interface between theoxide semiconductor films 19 a and 39 a. Thus, the transistor can havehigh field-effect mobility because the movement of carriers is nothindered at the interface.

The oxide semiconductor film 39 a is typically an In—Ga oxide film, anIn—Zn oxide film, or an In—M—Zn oxide film (M represents Al, Ga, Y, Zr,Sn, La, Ce, or Nd). The energy at the conduction band bottom of theoxide semiconductor film 39 a is closer to a vacuum level than that ofthe oxide semiconductor film 19 a is, and typically, the differencebetween the energy at the conduction band bottom of the oxidesemiconductor film 39 a and the energy at the conduction band bottom ofthe oxide semiconductor film 19 a is any one of 0.05 eV or more, 0.07 eVor more, 0.1 eV or more, or 0.15 eV or more, and any one of 2 eV orless, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. That is, thedifference between the electron affinity of the oxide semiconductor film39 a and the electron affinity of the oxide semiconductor film 19 a isany one of 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eVor more, and any one of 2 eV or less, 1 eV or less, 0.5 eV or less, or0.4 eV or less.

The oxide semiconductor film 39 a preferably contains In because carriermobility (electron mobility) can be increased.

When the oxide semiconductor film 39 a contains a larger amount of Al,Ga, Y, Zr, Sn, La, Ce, or Nd in an atomic ratio than the amount of In inan atomic ratio, any of the following effects may be obtained: (1) theenergy gap of the oxide semiconductor film 39 a is widened; (2) theelectron affinity of the oxide semiconductor films film 39 a is reduced;(3) scattering of impurities from the outside is reduced; (4) aninsulating property increases as compared to the oxide semiconductorfilm 19 a; and (5) oxygen vacancies are less likely to be generatedbecause Al, Ga, Y, Zr, Sn, La, Ce, or Nd is a metal element stronglybonded to oxygen.

In the case where the oxide semiconductor film 39 a is an In—M—Zn oxidefilm, the proportions of In and M when the summation of In and M isassumed to be 100 atomic % are preferably as follows: the atomicpercentage of In is less than 50 atomic % and the atomic percentage of Mis greater than 50 atomic %; further preferably, the atomic percentageof In is less than 25 atomic % and the atomic percentage of M is greaterthan 75 atomic %.

Furthermore, in the case where each of the oxide semiconductor films 19a and 39 a is an In—M—Zn oxide film (M represents Al, Ga, Y, Zr, Sn, La,Ce, or Nd), the proportion of M atoms (M represents Al, Ga, Y, Zr, Sn,La, Ce, or Nd) in the oxide semiconductor film 39 a is higher than thatin the oxide semiconductor film 19 a. As a typical example, theproportion of Min the oxide semiconductor film 39 a is 1.5 times ormore, preferably twice or more, further preferably three times or moreas high as that in the oxide semiconductor film 19 a.

Furthermore, in the case where each of the oxide semiconductor film 19 aand the oxide semiconductor film 39 a is an In—M—Zn oxide film (Mrepresents Al, Ga, Y, Zr, Sn, La, Ce, or Nd), when In:M:Zn=x₁:y₁:z₁[atomic ratio] is satisfied in the oxide semiconductor film 39 a andIn:M:Zn=x₂:y₂:z₂ [atomic ratio] is satisfied in the oxide semiconductorfilm 19 a, y₁/x₁ is higher than y₂/x₂. Preferably, y₁/x₁ is 1.5 times ormore as high as y₂/x₂. Further preferably, y₁/x₁ is twice or more ashigh as y₂/x₂. Still further preferably, y₁/x₁ is three times or more ashigh as y₂/x₂.

In the case where the oxide semiconductor film 19 a is an In—M—Zn oxidefilm (M is Al, Ga, Y, Zr, Sn, La, Ce, or Nd) and a target having theatomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for formingthe oxide semiconductor film 19 a, x₁/y₁ is preferably greater than orequal to ⅓ and less than or equal to 6, further preferably greater thanor equal to 1 and less than or equal to 6, and z₁/y₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, furtherpreferably greater than or equal to 1 and less than or equal to 6. Notethat when z₁/y₁ is greater than or equal to 1 and less than or equal to6, a CAAC-OS film to be described later as the oxide semiconductor film19 a is easily formed. Typical examples of the atomic ratio of the metalelements of the target are In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, andIn:M:Zn=3:1:2.

In the case where the oxide semiconductor film 39 a is an In—M—Zn oxidefilm (M is Al, Ga, Y, Zr, Sn, La, Ce, or Nd) and a target having theatomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used for formingthe oxide semiconductor film 39 a, x₂/y₂ is preferably less than x₁/y₁,and z₂/y₂ is preferably greater than or equal to ⅓ and less than orequal to 6, further preferably greater than or equal to 1 and less thanor equal to 6. Note that when z₂/y₂ is greater than or equal to 1 andless than or equal to 6, a CAAC-OS film to be described later as theoxide semiconductor film 39 a is easily formed. Typical examples of theatomic ratio of the metal elements of the target are In:M:Zn=1:3:2,In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8, In:M:Zn=1:4:4,In:M:Zn=1:4:5, In:M:Zn=1:4:6, In:M:Zn=1:4:7, In:M:Zn=1:4:8,In:M:Zn=1:5:5, In:M:Zn=1:5:6, In:M:Zn=1:5:7, In:M:Zn=1:5:8, andIn:M:Zn=1:6:8.

Note that the proportion of each metal element in the atomic ratio ofeach of the oxide semiconductor films 19 a and the oxide semiconductorfilm 39 a varies within a range of ±40% of that in the above atomicratio as an error.

The oxide semiconductor film 39 a also functions as a film that relievesdamage to the oxide semiconductor film 19 a at the time of forming theoxide insulating film 25 later.

The thickness of the oxide semiconductor film 39 a is greater than orequal to 3 nm and less than or equal to 100 nm, preferably greater thanor equal to 3 nm and less than or equal to 50 nm.

Furthermore, the oxide semiconductor film 39 a can have a crystalstructure of the oxide semiconductor film 19 a as appropriate.

Note that the oxide semiconductor films 19 a and 39 a may each be amixed film including two or more of the following: a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure. The mixed film has asingle-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Furthermore, in some cases, the mixed film has a stacked-layer structurein which two or more of the following regions are stacked: a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure.

Here, the oxide semiconductor film 39 a is formed between the oxidesemiconductor film 19 a and the oxide insulating film 23. Thus, ifcarrier traps are formed between the oxide semiconductor film 39 a andthe oxide insulating film 23 by impurities and defects, electronsflowing in the oxide semiconductor film 19 a are less likely to becaptured by the carrier traps because there is a distance between thecarrier traps and the oxide semiconductor film 19 a. Accordingly, theamount of on-state current of the transistor can be increased, and thefield-effect mobility can be increased. When the electrons are capturedby the carrier traps, the electrons become negative fixed charges. As aresult, a threshold voltage of the transistor changes. However, by thedistance between the oxide semiconductor film 19 a and the carriertraps, capture of electrons by the carrier traps can be reduced, andaccordingly, the amount of change in the threshold voltage can bereduced.

Impurities from the outside can be blocked by the oxide semiconductorfilm 39 a, and accordingly, the amount of impurities that aretransferred from the outside to the oxide semiconductor film 19 a can bereduced. Furthermore, an oxygen vacancy is less likely to be formed inthe oxide semiconductor film 39 a. Consequently, the impurityconcentration and the number of oxygen vacancies in the oxidesemiconductor film 19 a can be reduced.

Note that the oxide semiconductor films 19 a and 39 a are not onlyformed by simply stacking each film, but also are formed to have acontinuous junction (here, in particular, a structure in which theenergy of the bottom of the conduction band is changed continuouslybetween each film). In other words, a stacked-layer structure in whichthere exist no impurity that forms a defect level such as a trap centeror a recombination center at the interface between the films isprovided. If an impurity exists between the oxide semiconductor films 19a and 39 a which are stacked, a continuity of the energy band isdamaged, and the carrier is captured or recombined at the interface andthen disappears.

In order to form such a continuous energy band, it is necessary to formfilms continuously without being exposed to air, with use of amulti-chamber deposition apparatus (sputtering apparatus) including aload lock chamber. Each chamber in the sputtering apparatus ispreferably evacuated to be a high vacuum state (to the degree of about5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump suchas a cryopump in order to remove water or the like, which serves as animpurity against the oxide semiconductor film, as much as possible.Alternatively, a turbo molecular pump and a cold trap are preferablycombined so as to prevent a backflow of a gas, especially a gascontaining carbon or hydrogen from an exhaust system to the inside ofthe chamber.

As in a transistor 102 c in FIG. 34B, a multilayer film 38 a may beprovided instead of the multilayer film 37 a.

In addition, as in a capacitor 105 c in FIG. 34B, a multilayer film 38 bmay be provided instead of the multilayer film 37 b.

The multilayer film 38 a includes an oxide semiconductor film 49 a, theoxide semiconductor film 19 a, and the oxide semiconductor film 39 a.That is, the multilayer film 38 a has a three-layer structure.Furthermore, the oxide semiconductor film 19 a functions as a channelregion.

The oxide semiconductor film 49 a can be formed using a material and aformation method similar to those of the oxide semiconductor film 39 a.

The multilayer film 38 b includes an oxide semiconductor film 49 bhaving conductivity, an oxide semiconductor film 19 f havingconductivity, and an oxide semiconductor film 39 b having conductivity.In other words, the multilayer film 38 b has a three-layer structure.The multilayer film 38 b functions as a pixel electrode.

The oxide semiconductor film 49 b can be formed using a material and aformation method similar to those of the oxide semiconductor film 39 bas appropriate.

In addition, the oxide insulating film 17 and the oxide semiconductorfilm 49 a are in contact with each other. That is, the oxidesemiconductor film 49 a is provided between the oxide insulating film 17and the oxide semiconductor film 19 a.

The multilayer film 38 a and the oxide insulating film 23 are in contactwith each other. In addition, the oxide semiconductor film 39 a and theoxide insulating film 23 are in contact with each other. That is, theoxide semiconductor film 39 a is provided between the oxidesemiconductor film 19 a and the oxide insulating film 23.

It is preferable that the thickness of the oxide semiconductor film 49 abe smaller than that of the oxide semiconductor film 19 a. When thethickness of the oxide semiconductor film 49 a is greater than or equalto 1 nm and less than or equal to 5 nm, preferably greater than or equalto 1 nm and less than or equal to 3 nm, the amount of change in thethreshold voltage of the transistor can be reduced.

In the transistor described in this embodiment, the oxide semiconductorfilm 39 a is provided between the oxide semiconductor film 19 a and theoxide insulating film 23. Thus, if carrier traps are formed between theoxide semiconductor film 39 a and the oxide insulating film 23 byimpurities and defects, electrons flowing in the oxide semiconductorfilm 19 a are less likely to be captured by the carrier traps becausethere is a distance between the carrier traps and the oxidesemiconductor film 19 a. Accordingly, the amount of on-state current ofthe transistor can be increased, and the field-effect mobility can beincreased. When the electrons are captured by the carrier traps, theelectrons become negative fixed charges. As a result, a thresholdvoltage of the transistor changes. However, by the distance between theoxide semiconductor film 19 a and the carrier traps, capture ofelectrons by the carrier traps can be reduced, and accordingly, theamount of change in the threshold voltage can be reduced.

Impurities from the outside can be blocked by the oxide semiconductorfilm 39 a, and accordingly, the amount of impurities that aretransferred from the outside to the oxide semiconductor film 19 a can bereduced. Furthermore, an oxygen vacancy is less likely to be formed inthe oxide semiconductor film 39 a. Consequently, the impurityconcentration and the number of oxygen vacancies in the oxidesemiconductor film 19 a can be reduced.

Furthermore, the oxide semiconductor film 49 a is provided between theoxide insulating film 17 and the oxide semiconductor film 19 a, and theoxide semiconductor film 39 a is provided between the oxidesemiconductor film 19 a and the oxide insulating film 23. Thus, it ispossible to reduce the concentration of silicon or carbon in thevicinity of the interface between the oxide semiconductor film 49 a andthe oxide semiconductor film 19 a, the concentration of silicon orcarbon in the oxide semiconductor film 19 a, or the concentration ofsilicon or carbon in the vicinity of the interface between the oxidesemiconductor film 39 a and the oxide semiconductor film 19 a.Consequently, in the multilayer film 38 a, the absorption coefficientderived from a constant photocurrent method is lower than 1×10⁻³/cm,preferably lower than 1×10⁻⁴/cm, and thus density of localized levels isextremely low.

The transistor 102 c having such a structure includes very few defectsin the multilayer film 38 a including the oxide semiconductor film 19 a;thus, the electrical characteristics of the transistor can be improved,and typically, the on-state current can be increased and thefield-effect mobility can be improved. Moreover, in a BT stress test anda BT photostress test which are examples of a stress test, the amount ofchange in threshold voltage is small, and thus, reliability is high.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 8

In this embodiment, a light-emitting device provided with part of theelement layer that is formed over the first substrate 11 in Embodiments4 to 7 is described with reference to FIGS. 35 and 36. Note that here,part of the element layer described in Embodiments 4 and 5 is used;however, an element layer having another structure can be used in thelight-emitting device as appropriate.

A light-emitting device in FIG. 35 includes, in addition to the elementlayer formed over the first substrate 11 in FIG. 17 of Embodiment 4, aninsulating film 371 provided over the inorganic insulating film 30, anEL layer 373 provided over the inorganic insulating film 30 and theoxide semiconductor film 19 b having conductivity, and a conductive film375 provided over the EL layer 373 and the insulating film 371. Theoxide semiconductor film 19 b having conductivity, the EL layer 373, andthe conductive film 375 constitute a light-emitting element 370 a.

A light-emitting device in FIG. 36 includes, in addition to the elementlayer formed over the first substrate 11 in FIG. 28 of Embodiment 5, theinsulating film 371 provided over the inorganic insulating film 30 andthe light-transmitting conductive film 29 c, the EL layer 373 providedover the inorganic insulating film 30 and the light-transmittingconductive film 29 c, and the conductive film 375 provided over the ELlayer 373 and the insulating film 371. The light-transmitting conductivefilm 29 c, the EL layer 373, and the conductive film 375 constitute alight-emitting element 370 b.

On the element substrate of the light-emitting device in thisembodiment, the oxide semiconductor film having conductivity serving asthe pixel electrode is formed at the same time as the oxidesemiconductor film of the transistor. Thus, the light-emitting devicecan be manufactured through fewer steps than the conventional case.

Alternatively, on the element substrate of the light-emitting device inthis embodiment, the oxide semiconductor film having conductivityserving as the electrode of the capacitor is formed at the same time asthe oxide semiconductor film of the transistor. The oxide semiconductorfilm having conductivity serves as one electrode of the capacitor. Thus,a step of forming another conductive film is not needed to form thecapacitor, and the number of steps of manufacturing the light-emittingdevice can be reduced. Furthermore, the other electrode of the capacitoris formed using a light-transmitting conductive film serving as anelectrode. Thus, the capacitor has light-transmitting properties. As aresult, the area occupied by the capacitor can be increased and theaperture ratio in a pixel can be increased.

The structures, methods, and the like described in this embodiment canbe used as appropriate in combination with any of the structures,methods, and the like described in the other embodiments.

Embodiment 9

In this embodiment, one embodiment that can be applied to an oxidesemiconductor film in the transistor included in the display devicedescribed in the above embodiment is described.

<Structure of Oxide Semiconductor>

A structure of an oxide semiconductor is described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a microcrystalline oxide semiconductor, and an amorphousoxide semiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and a microcrystalline oxide semiconductor.

<CAAC-OS>

First, a CAAC-OS is described. Note that a CAAC-OS can be referred to asan oxide semiconductor including c-axis aligned nanocrystals (CANC).

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

A CAAC-OS observed with TEM is described below. FIG. 37A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 37B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 37A. FIG. 37B shows that metal atoms are arranged ina layered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which the CAAC-OS is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 37B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 37C. FIGS. 37B and 37C prove that the size of apellet is approximately 1 nm to 3 nm, and the size of a space caused bytilt of the pellets is approximately 0.8 nm. Therefore, the pellet canalso be referred to as a nanocrystal (nc).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 37D). The part in which the pellets are tilted as observed inFIG. 37C corresponds to a region 5161 shown in FIG. 37D.

FIG. 38A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 38B, 38C, and 38D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 38A,respectively. FIGS. 38B, 38C, and 38D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalis analyzed by an out-of-plane method, a peak appears at a diffractionangle (2θ) of around 31° as shown in FIG. 39A. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak at 2θ of around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is attributed to the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (φ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (φ axis), as shown in FIG. 39B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when φ scan is performed with2θ fixed at around 56°, as shown in FIG. 39C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are different in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 40A might be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 40B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 40B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 40B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.The second ring in FIG. 40B is considered to be derived from the (110)plane and the like.

Moreover, the CAAC-OS is an oxide semiconductor having a low density ofdefect states. Defects in the oxide semiconductor are, for example, adefect due to impurity and oxygen vacancies. Therefore, the CAAC-OS canbe regarded as an oxide semiconductor with a low impurity concentration,or an oxide semiconductor having a small number of oxygen vacancies.

The impurity contained in the oxide semiconductor might serve as acarrier trap or serve as a carrier generation source. Furthermore,oxygen vacancies in the oxide semiconductor serve as carrier traps orserve as carrier generation sources when hydrogen is captured therein.

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

An oxide semiconductor having a low density of defect states (a smallnumber of oxygen vacancies) can have a low carrier density. Such anoxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.That is, a CAAC-OS is likely to be highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. Thus, atransistor including a CAAC-OS rarely has negative threshold voltage (israrely normally on). The highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor has few carrier traps. Anelectric charge trapped by the carrier traps in the oxide semiconductortakes a long time to be released. The trapped electric charge may behavelike a fixed electric charge. Thus, the transistor which includes theoxide semiconductor having a high impurity concentration and a highdensity of defect states might have unstable electrical characteristics.However, a transistor including a CAAC-OS has small variation inelectrical characteristics and high reliability.

Since the CAAC-OS has a low density of defect states, carriers generatedby light irradiation or the like are less likely to be trapped in defectstates. Therefore, in a transistor using the CAAC-OS, change inelectrical characteristics due to irradiation with visible light orultraviolet light is small.

<Microcrystalline Oxide Semiconductor>

Next, a microcrystalline oxide semiconductor is described.

A microcrystalline oxide semiconductor has a region in which a crystalpart is observed and a region in which a crystal part is not clearlyobserved in a high-resolution TEM image. In most cases, the size of acrystal part included in the microcrystalline oxide semiconductor isgreater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. An oxidesemiconductor including a nanocrystal (nc) that is a microcrystal with asize greater than or equal to 1 nm and less than or equal to 10 nm, or asize greater than or equal to 1 nm and less than or equal to 3 nm isspecifically referred to as a nanocrystalline oxide semiconductor(nc-OS). In a high-resolution TEM image of the nc-OS, for example, agrain boundary is not clearly observed in some cases. Note that there isa possibility that the origin of the nanocrystal is the same as that ofa pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may bereferred to as a pellet in the following description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS cannot be distinguished from an amorphous oxidesemiconductor, depending on an analysis method. For example, when thenc-OS is subjected to structural analysis by an out-of-plane method withan XRD apparatus using an X-ray having a diameter larger than the sizeof a pellet, a peak which shows a crystal plane does not appear.Furthermore, a diffraction pattern like a halo pattern is observed whenthe nc-OS is subjected to electron diffraction using an electron beamwith a probe diameter (e.g., 50 nm or larger) that is larger than thesize of a pellet (the electron diffraction is also referred to asselected-area electron diffraction). Meanwhile, spots appear in ananobeam electron diffraction pattern of the nc-OS when an electron beamhaving a probe diameter close to or smaller than the size of a pellet isapplied. Moreover, in a nanobeam electron diffraction pattern of thenc-OS, regions with high luminance in a circular (ring) pattern areshown in some cases. Also in a nanobeam electron diffraction pattern ofthe nc-OS, a plurality of spots is shown in a ring-like region in somecases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an amorphous oxidesemiconductor. Note that there is no regularity of crystal orientationbetween different pellets in the nc-OS. Therefore, the nc-OS has ahigher density of defect states than the CAAC-OS.

<Amorphous Oxide Semiconductor>

Next, an amorphous oxide semiconductor is described.

The amorphous oxide semiconductor is an oxide semiconductor havingdisordered atomic arrangement and no crystal part and exemplified by anoxide semiconductor which exists in an amorphous state as quartz.

In a high-resolution TEM image of the amorphous oxide semiconductor,crystal parts cannot be found.

When the amorphous oxide semiconductor is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is observed whenthe amorphous oxide semiconductor is subjected to electron diffraction.Furthermore, a spot is not observed and only a halo pattern appears whenthe amorphous oxide semiconductor is subjected to nanobeam electrondiffraction.

There are various understandings of an amorphous structure. For example,a structure whose atomic arrangement does not have ordering at all iscalled a completely amorphous structure. Meanwhile, a structure whichhas ordering until the nearest neighbor atomic distance or thesecond-nearest neighbor atomic distance but does not have long-rangeordering is also called an amorphous structure. Therefore, the strictestdefinition does not permit an oxide semiconductor to be called anamorphous oxide semiconductor as long as even a negligible degree ofordering is present in an atomic arrangement. At least an oxidesemiconductor having long-term ordering cannot be called an amorphousoxide semiconductor. Accordingly, because of the presence of crystalpart, for example, a CAAC-OS and an nc-OS cannot be called an amorphousoxide semiconductor or a completely amorphous oxide semiconductor.

<Amorphous-Like Oxide Semiconductor>

Note that an oxide semiconductor may have a structure intermediatebetween the nc-OS and the amorphous oxide semiconductor. The oxidesemiconductor having such a structure is specifically referred to as anamorphous-like oxide semiconductor (a-like OS).

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it includes a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (sample A), an nc-OS (sample B), and a CAAC-OS (sample C)are prepared as samples subjected to electron irradiation. Each of thesamples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of an InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 41 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 41 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 41, acrystal part of approximately 1.2 nm (also referred to as an initialnucleus) at the start of TEM observation grows to a size ofapproximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². Incontrast, the crystal part size in the nc-OS and the CAAC-OS showslittle change from the start of electron irradiation to a cumulativeelectron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by (2) and (3)in FIG. 41, the average crystal sizes in an nc-OS and a CAAC-OS areapproximately 1.4 nm and approximately 2.1 nm, respectively, regardlessof the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit includes a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more films of an amorphous oxide semiconductor,an a-like OS, a microcrystalline oxide semiconductor, and a CAAC-OS, forexample.

<Deposition Model>

Examples of deposition models of a CAAC-OS and an nc-OS are describedbelow.

FIG. 42A is a schematic view of the inside of a deposition chamber wherea CAAC-OS is deposited by a sputtering method.

A target 5130 is attached to a backing plate. A plurality of magnets isprovided to face the target 5130 with the backing plate positionedtherebetween. The plurality of magnets generates a magnetic field. Asputtering method in which the disposition rate is increased byutilizing a magnetic field of magnets is referred to as a magnetronsputtering method.

The substrate 5120 is placed to face the target 5130, and the distance d(also referred to as a target-substrate distance (T-S distance)) isgreater than or equal to 0.01 m and less than or equal to 1 m,preferably greater than or equal to 0.02 m and less than or equal to 0.5m. The deposition chamber is mostly filled with a deposition gas (e.g.,an oxygen gas, an argon gas, or a mixed gas containing oxygen at 5 vol %or higher) and the pressure in the deposition chamber is controlled tobe higher than or equal to 0.01 Pa and lower than or equal to 100 Pa,preferably higher than or equal to 0.1 Pa and lower than or equal to 10Pa. Here, discharge starts by application of a voltage at a certainvalue or higher to the target 5130, and plasma is observed. The magneticfield forms a high-density plasma region in the vicinity of the target5130. In the high-density plasma region, the deposition gas is ionized,so that an ion 5101 is generated. Examples of the ion 5101 include anoxygen cation (O⁺) and an argon cation (Ar⁺).

Here, the target 5130 has a polycrystalline structure which includes aplurality of crystal grains and in which a cleavage plane exists in atleast one crystal grain. FIG. 43A shows a structure of an InGaZnO₄crystal included in the target 5130 as an example. Note that FIG. 43Ashows a structure of the case where the InGaZnO₄ crystal is observedfrom a direction parallel to the b-axis. FIG. 43A indicates that oxygenatoms in a Ga—Zn—O layer are positioned close to those in an adjacentGa—Zn—O layer. The oxygen atoms have negative charge, whereby repulsiveforce is generated between the two adjacent Ga—Zn—O layers. As a result,the InGaZnO₄ crystal has a cleavage plane between the two adjacentGa—Zn—O layers.

The ion 5101 generated in the high-density plasma region is acceleratedtoward the target 5130 side by an electric field, and then collides withthe target 5130. At this time, a pellet 5100 a and a pellet 5100 b whichare flat-plate-like (pellet-like) sputtered particles are separated andsputtered from the cleavage plane. Note that structures of the pellet5100 a and the pellet 5100 b may be distorted by an impact of collisionof the ion 5101.

The pellet 5100 a is a flat-plate-like (pellet-like) sputtered particlehaving a triangle plane, e.g., regular triangle plane. The pellet 5100 bis a flat-plate-like (pellet-like) sputtered particle having a hexagonplane, e.g., regular hexagon plane. Note that flat-plate-like(pellet-like) sputtered particles such as the pellet 5100 a and thepellet 5100 b are collectively called pellets 5100. The shape of a flatplane of the pellet 5100 is not limited to a triangle or a hexagon. Forexample, the flat plane may have a shape formed by combining two or moretriangles. For example, a quadrangle (e.g., rhombus) may be formed bycombining two triangles (e.g., regular triangles).

The thickness of the pellet 5100 is determined depending on the kind ofdeposition gas and the like. The thicknesses of the pellets 5100 arepreferably uniform; the reason for this is described later. In addition,the sputtered particle preferably has a pellet shape with a smallthickness as compared to a dice shape with a large thickness. Forexample, the thickness of the pellet 5100 is greater than or equal to0.4 nm and less than or equal to 1 nm, preferably greater than or equalto 0.6 nm and less than or equal to 0.8 nm. In addition, for example,the width of the pellet 5100 is greater than or equal to 1 nm and lessthan or equal to 3 nm, preferably greater than or equal to 1.2 nm andless than or equal to 2.5 nm. The pellet 5100 corresponds to the initialnucleus in the description of (1) in FIG. 41. For example, when the ion5101 collides with the target 5130 including an In—Ga—Zn oxide, thepellet 5100 that includes three layers of a Ga—Zn—O layer, an In—Olayer, and a Ga—Zn—O layer as shown in FIG. 43B is separated. Note thatFIG. 43C shows the structure of the separated pellet 5100 which isobserved from a direction parallel to the c-axis. The pellet 5100 has ananometer-sized sandwich structure including two Ga—Zn—O layers and anIn—O layer.

The pellet 5100 may receive a charge when passing through the plasma, sothat side surfaces thereof are negatively or positively charged. In thepellet 5100, for example, an oxygen atom positioned on its side surfacemay be negatively charged. When the side surfaces are charged with thesame polarity, charges repel each other, and accordingly, the pellet5100 can maintain a flat-plate (pellet) shape. In the case where aCAAC-OS is an In—Ga—Zn oxide, there is a possibility that an oxygen atombonded to an indium atom is negatively charged. There is anotherpossibility that an oxygen atom bonded to an indium atom, a galliumatom, or a zinc atom is negatively charged. In addition, the pellet 5100may grow by being bonded with an indium atom, a gallium atom, a zincatom, an oxygen atom, or the like when passing through plasma. Adifference in size between (2) and (1) in FIG. 41 corresponds to theamount of growth in plasma. Here, in the case where the temperature ofthe substrate 5120 is at around room temperature, the pellet 5100 on thesubstrate 5120 hardly grows; thus, an nc-OS is formed (see FIG. 42B). Annc-OS can be deposited when the substrate 5120 has a large size becausethe deposition of an nc-OS can be carried out at room temperature. Notethat in order that the pellet 5100 grows in plasma, it is effective toincrease deposition power in sputtering. High deposition power canstabilize the structure of the pellet 5100.

As shown in FIGS. 42A and 42B, the pellet 5100 flies like a kite inplasma and flutters up to the substrate 5120. Since the pellets 5100 arecharged, when the pellet 5100 gets close to a region where anotherpellet 5100 has already been deposited, repulsion is generated. Here,above the substrate 5120, a magnetic field in a direction parallel tothe top surface of the substrate 5120 (also referred to as a horizontalmagnetic field) is generated. A potential difference is given betweenthe substrate 5120 and the target 5130, and accordingly, current flowsfrom the substrate 5120 toward the target 5130. Thus, the pellet 5100 isgiven a force (Lorentz force) on the top surface of the substrate 5120by an effect of the magnetic field and the current. This is explainablewith Fleming's left-hand rule.

The mass of the pellet 5100 is larger than that of an atom. Therefore,to move the pellet 5100 over the top surface of the substrate 5120, itis important to apply some force to the pellet 5100 from the outside.One kind of the force may be force which is generated by the action of amagnetic field and current. In order to apply a sufficient force to thepellet 5100 so that the pellet 5100 moves over a top surface of thesubstrate 5120, it is preferable to provide, on the top surface, aregion where the magnetic field in a direction parallel to the topsurface of the substrate 5120 is 10 G or higher, preferably 20 G orhigher, further preferably 30 G or higher, still further preferably 50 Gor higher. Alternatively, it is preferable to provide, on the topsurface, a region where the magnetic field in a direction parallel tothe top surface of the substrate 5120 is 1.5 times or higher, preferablytwice or higher, further preferably 3 times or higher, still furtherpreferably 5 times or higher as high as the magnetic field in adirection perpendicular to the top surface of the substrate 5120.

At this time, the magnets and the substrate 5120 are moved or rotatedrelatively, whereby the direction of the horizontal magnetic field onthe top surface of the substrate 5120 continues to change. Therefore,the pellet 5100 can be moved in various directions on the top surface ofthe substrate 5120 by receiving forces in various directions.

Furthermore, as shown in FIG. 42A, when the substrate 5120 is heated,resistance between the pellet 5100 and the substrate 5120 due tofriction or the like is low. As a result, the pellet 5100 glides abovethe top surface of the substrate 5120. The glide of the pellet 5100 iscaused in a state where its flat plane faces the substrate 5120. Then,when the pellet 5100 reaches the side surface of another pellet 5100that has been already deposited, the side surfaces of the pellets 5100are bonded. At this time, the oxygen atom on the side surface of thepellet 5100 is released. With the released oxygen atom, oxygen vacanciesin a CAAC-OS might be filled; thus, the CAAC-OS has a low density ofdefect states. Note that the temperature of the top surface of thesubstrate 5120 is, for example, higher than or equal to 100° C. andlower than 500° C., higher than or equal to 150° C. and lower than 450°C., or higher than or equal to 170° C. and lower than 400° C. Hence,even when the substrate 5120 has a large size, it is possible to deposita CAAC-OS.

Furthermore, the pellet 5100 is heated on the substrate 5120, wherebyatoms are rearranged, and the structure distortion caused by thecollision of the ion 5101 can be reduced. The pellet 5100 whosestructure distortion is reduced is substantially single crystal. Evenwhen the pellets 5100 are heated after being bonded, expansion andcontraction of the pellet 5100 itself hardly occur, which is caused byturning the pellet 5100 into substantially single crystal. Thus,formation of defects such as a grain boundary due to expansion of aspace between the pellets 5100 can be prevented, and accordingly,generation of crevasses can be prevented.

The CAAC-OS does not have a structure like a board of a single crystaloxide semiconductor but has arrangement with a group of pellets 5100(nanocrystals) like stacked bricks or blocks. Furthermore, a grainboundary does not exist between the pellets 5100. Therefore, even whendeformation such as shrink occurs in the CAAC-OS owing to heating duringdeposition, heating or bending after deposition, it is possible torelieve local stress or release distortion. Therefore, this structure issuitable for a flexible semiconductor device. Note that the nc-OS hasarrangement in which pellets 5100 (nanocrystals) are randomly stacked.

When the target 5130 is sputtered with the ion 5101, in addition to thepellets 5100, zinc oxide or the like may be separated. The zinc oxide islighter than the pellet 5100 and thus reaches the top surface of thesubstrate 5120 before the pellet 5100. As a result, the zinc oxide formsa zinc oxide layer 5102 with a thickness greater than or equal to 0.1 nmand less than or equal to 10 nm, greater than or equal to 0.2 nm andless than or equal to 5 nm, or greater than or equal to 0.5 nm and lessthan or equal to 2 nm. FIGS. 44A to 44D are cross-sectional schematicviews.

As illustrated in FIG. 44A, a pellet 5105 a and a pellet 5105 b aredeposited over the zinc oxide layer 5102. Here, side surfaces of thepellet 5105 a and the pellet 5105 b are in contact with each other. Inaddition, a pellet 5105 c is deposited over the pellet 5105 b, and thenglides over the pellet 5105 b. Furthermore, a plurality of particles5103 separated from the target together with the zinc oxide iscrystallized by heat from the substrate 5120 to form a region 5105 a 1on another side surface of the pellet 5105 a. Note that the plurality ofparticles 5103 may contain oxygen, zinc, indium, gallium, or the like.

Then, as illustrated in FIG. 44B, the region 5105 a 1 grows to part ofthe pellet 5105 a to form a pellet 5105 a 2. In addition, a side surfaceof the pellet 5105 c is in contact with another side surface of thepellet 5105 b.

Next, as illustrated in FIG. 44C, a pellet 5105 d is deposited over thepellet 5105 a 2 and the pellet 5105 b, and then glides over the pellet5105 a 2 and the pellet 5105 b. Furthermore, a pellet 5105 e glidestoward another side surface of the pellet 5105 c over the zinc oxidelayer 5102.

Then, as illustrated in FIG. 44D, the pellet 5105 d is placed so that aside surface of the pellet 5105 d is in contact with a side surface ofthe pellet 5105 a 2. Furthermore, a side surface of the pellet 5105 e isin contact with another side surface of the pellet 5105 c. A pluralityof particles 5103 separated from the target 5130 together with the zincoxide is crystallized by heat from the substrate 5120 to form a region5105 d 1 on another side surface of the pellet 5105 d.

As described above, deposited pellets are placed to be in contact witheach other and then growth is caused at side surfaces of the pellets,whereby a CAAC-OS is formed over the substrate 5120. Therefore, eachpellet of the CAAC-OS is larger than that of the nc-OS. A difference insize between (3) and (2) in FIG. 41 corresponds to the amount of growthafter deposition.

When spaces between pellets are extremely small, the pellets may form alarge pellet. The large pellet has a single crystal structure. Forexample, the size of the pellet may be greater than or equal to 10 nmand less than or equal to 200 nm, greater than or equal to 15 nm andless than or equal to 100 nm, or greater than or equal to 20 nm and lessthan or equal to 50 nm, when seen from the above. In this case, in anoxide semiconductor used for a minute transistor, a channel formationregion might be fit inside the large pellet. That is, the region havinga single crystal structure can be used as the channel formation region.Furthermore, when the size of the pellet is increased, the region havinga single crystal structure can be used as the channel formation region,the source region, and the drain region of the transistor.

In this manner, when the channel formation region or the like of thetransistor is formed in a region having a single crystal structure, thefrequency characteristics of the transistor can be increased in somecases.

As shown in such a model, the pellets 5100 are considered to bedeposited on the substrate 5120. Thus, a CAAC-OS can be deposited evenwhen a formation surface does not have a crystal structure; therefore, agrowth mechanism in this case is different from epitaxial growth. Inaddition, laser crystallization is not needed for formation of aCAAC-OS, and a uniform film can be formed even over a large-sized glasssubstrate or the like. For example, even when the top surface (formationsurface) of the substrate 5120 has an amorphous structure (e.g., the topsurface is formed of amorphous silicon oxide), a CAAC-OS can be formed.

In addition, it is found that in formation of the CAAC-OS, the pellets5100 are arranged in accordance with the top surface shape of thesubstrate 5120 that is the formation surface even when the formationsurface has unevenness. For example, in the case where the top surfaceof the substrate 5120 is flat at the atomic level, the pellets 5100 arearranged so that flat planes parallel to the a-b plane face downwards.In the case where the thicknesses of the pellets 5100 are uniform, alayer with a uniform thickness, flatness, and high crystallinity isformed. By stacking n layers (n is a natural number), the CAAC-OS can beobtained.

In the case where the top surface of the substrate 5120 has unevenness,a CAAC-OS in which n layers (n is a natural number) in each of which thepellets 5100 are arranged along the unevenness are stacked is formed.Since the substrate 5120 has unevenness, a gap is easily generatedbetween the pellets 5100 in the CAAC-OS in some cases. Note that, evenin such a case, owing to intermolecular force, the pellets 5100 arearranged so that a gap between the pellets is as small as possible evenon the unevenness surface. Therefore, even when the formation surfacehas unevenness, a CAAC-OS with high crystallinity can be obtained.

Since a CAAC-OS is deposited in accordance with such a model, thesputtered particle preferably has a pellet shape with a small thickness.Note that when the sputtered particles have a dice shape with a largethickness, planes facing the substrate 5120 vary; thus, the thicknessesand orientations of the crystals cannot be uniform in some cases.

According to the deposition model described above, a CAAC-OS with highcrystallinity can be formed even on a formation surface with anamorphous structure.

<Cleavage Plane>

A cleavage plane that has been mentioned in the deposition model of theCAAC-OS is described below.

First, a cleavage plane of the target is described with reference toFIGS. 45A and 45B. FIGS. 45A and 45B show the crystal structure ofInGaZnO₄. Note that FIG. 45A shows the structure of the case where anInGaZnO₄ crystal is observed from a direction parallel to the b-axiswhen the c-axis is in an upward direction. Furthermore, FIG. 45B showsthe structure of the case where the InGaZnO₄ crystal is observed from adirection parallel to the c-axis.

Energy needed for cleavage at each crystal plane of the InGaZnO₄ crystalis calculated by the first principles calculation. Note that a pseudopotential and a density functional theory program (CASTEP) using theplane wave basis are used for the calculation. An ultrasoft type pseudopotential is used as the pseudo potential. Furthermore, GGA/PBE is usedas the functional. Cut-off energy is 400 eV.

Energy of a structure in an initial state is obtained after structuraloptimization including a cell size is performed. Furthermore, energy ofa structure after the cleavage at each plane is obtained afterstructural optimization of atomic order is performed in a state wherethe cell size is fixed.

On the basis of the structure of the InGaZnO₄ crystal in FIGS. 45A and45B, a structure cleaved at any one of a first plane, a second plane, athird plane, and a fourth plane is formed and subjected to structuraloptimization calculation in which the cell size is fixed. Here, thefirst plane is a crystal plane between a Ga—Zn—O layer and an In—O layerand is parallel to the (001) plane (or the a-b plane) (see FIG. 45A).The second plane is a crystal plane between a Ga—Zn—O layer and aGa—Zn—O layer and is parallel to the (001) plane (or the a-b plane) (seeFIG. 45A). The third plane is a crystal plane parallel to the (110)plane (see FIG. 45B). The fourth plane is a crystal plane parallel tothe (100) plane (or the b-c plane) (see FIG. 45B).

Under the above conditions, the energy of the structure at each planeafter the cleavage is calculated. Next, a difference between the energyof the structure after the cleavage and the energy of the structure inthe initial state is divided by the area of the cleavage plane; thus,cleavage energy that serves as a measure of easiness of cleavage at eachplane is calculated. Note that the energy of a structure is calculatedbased on atoms and electrons included in the structure. That is, kineticenergy of the electrons and interactions between the atoms, between theatom and the electron, and between the electrons are considered in thecalculation.

As calculation results, the cleavage energy of the first plane is 2.60J/m², that of the second plane is 0.68 J/m², that of the third plane is2.18 J/m², and that of the fourth plane is 2.12 J/m² (see Table 1).

TABLE 1 Cleavage energy [J/m²] First plane 2.60 Second plane 0.68 Thirdplane 2.18 Fourth plane 2.12

From the calculations, in the structure of the InGaZnO₄ crystal in FIGS.45A and 45B, the cleavage energy of the second plane is the lowest. Inother words, a plane between a Ga—Zn—O layer and a Ga—Zn—O layer iscleaved most easily (cleavage plane). Therefore, in this specification,the cleavage plane indicates the second plane, which is a plane wherecleavage is performed most easily.

Since the cleavage plane is the second plane between the Ga—Zn—O layerand the Ga—Zn—O layer, the InGaZnO₄ crystals in FIG. 45A can beseparated at a plane equivalent to two second planes. Thus, in the casewhere an ion or the like is made to collide with a target, a wafer-likeunit (we call this a pellet) that is cleaved at a plane with the lowestcleavage energy is thought to be blasted off as the minimum unit. Inthat case, a pellet of InGaZnO₄ includes three layers: a Ga—Zn—O layer,an In—O layer, and a Ga—Zn—O layer.

The cleavage energies of the third plane (crystal plane parallel to the(110) plane) and the fourth plane (crystal plane parallel to the (100)plane (or the b-c plane)) are lower than that of the first plane(crystal plane between the Ga—Zn—O layer and the In—O layer and crystalplane parallel to the (001) plane (or the a-b plane)), which suggeststhat most of the flat planes of the pellets have triangle shapes orhexagonal shapes.

Next, through classical molecular dynamics calculation, on theassumption of an InGaZnO₄ crystal having a homologous structure as atarget, a cleavage plane is examined in the case where the target issputtered using argon (Ar) or oxygen (O). FIG. 46A shows across-sectional structure of an InGaZnO₄ crystal (2688 atoms) used forthe calculation, and FIG. 46B shows a top structure thereof. Note that afixed layer in FIG. 46A prevents the positions of the atoms from moving.A temperature control layer in FIG. 46A is a layer whose temperature isconstantly set to fixed temperature (300 K).

For the classical molecular dynamics calculation, Materials Explorer 5.0manufactured by Fujitsu Limited is used. Note that the initialtemperature, the cell size, the time step size, and the number of stepsare set to be 300 K, a certain size, 0.01 fs, and ten million,respectively. In calculation, an atom to which an energy of 300 eV isapplied is made to enter a cell from a direction perpendicular to thea-b plane of the InGaZnO₄ crystal under the conditions.

FIG. 47A shows atomic order when 99.9 picoseconds have passed afterargon enters the cell including the InGaZnO₄ crystal in FIGS. 46A and46B. FIG. 47B shows atomic order when 99.9 picoseconds have passed afteroxygen enters the cell. Note that in FIGS. 47A and 47B, part of thefixed layer in FIG. 46A is omitted.

According to FIG. 47A, in a period from entry of argon into the cell towhen 99.9 picoseconds have passed, a crack is formed from the cleavageplane corresponding to the second plane in FIG. 45A. Thus, in the casewhere argon collides with the InGaZnO₄ crystal and the uppermost surfaceis the second plane (the zero-th), a large crack is found to be formedin the second plane (the second).

On the other hand, according to FIG. 47B, in a period from entry ofoxygen into the cell to when 99.9 picoseconds have passed, a crack isfound to be formed from the cleavage plane corresponding to the secondplane in FIG. 45A. Note that in the case where oxygen collides with thecell, a large crack is found to be formed in the second plane (thefirst) of the InGaZnO₄ crystal.

Accordingly, it is found that an atom (ion) collides with a targetincluding an InGaZnO₄ crystal having a homologous structure from theupper surface of the target, the InGaZnO₄ crystal is cleaved along thesecond plane, and a flat-plate-like sputtered particle (pellet) isseparated. It is also found that the pellet formed in the case whereoxygen collides with the cell is smaller than that formed in the casewhere argon collides with the cell.

The above calculation suggests that the separated pellet includes adamaged region. In some cases, the damaged region included in the pelletcan be repaired in such a manner that a defect caused by the damagereacts with oxygen.

Here, a difference in size of the pellet depending on atoms that aremade to collide is studied.

FIG. 48A shows trajectories of the atoms from 0 picosecond to 0.3picoseconds after argon enters the cell including the InGaZnO₄ crystalin FIGS. 46A and 46B. Accordingly, FIG. 48A corresponds to a period fromFIGS. 46A and 46B to FIG. 47A.

According to FIG. 48A, when argon collides with gallium (Ga) of thefirst layer (Ga—Zn—O layer), gallium collides with zinc (Zn) of thethird layer (Ga—Zn—O layer) and then, zinc reaches the vicinity of thesixth layer (Ga—Zn—O layer). Note that argon which collides with galliumis sputtered to the outside. Accordingly, in the case where argoncollides with the target including the InGaZnO₄ crystal, a crack isthought to be formed in the second plane (the second) in FIG. 46A.

FIG. 48B shows trajectories of the atoms from 0 picosecond to 0.3picoseconds after oxygen enters the cell including the InGaZnO₄ crystalin FIGS. 46A and 46B. Accordingly, FIG. 48B corresponds to a period fromFIGS. 46A and 46B to FIG. 47A.

On the other hand, according to FIG. 48B, when oxygen collides withgallium (Ga) of the first layer (Ga—Zn—O layer), gallium collides withzinc (Zn) of the third layer (Ga—Zn—O layer) and then, zinc does notreach the fifth layer (In—O layer). Note that oxygen which collides withgallium is sputtered to the outside. Accordingly, in the case whereoxygen collides with the target including the InGaZnO₄ crystal, a crackis thought to be formed in the second plane (the first) in FIG. 46A.

This calculation also shows that the InGaZnO₄ crystal with which an atom(ion) collides is separated from the cleavage plane.

In addition, a difference in depth of a crack is examined in view ofconservation laws. The energy conservation law and the law ofconservation of momentum can be represented by Formula (1) and Formula(2). Here, E represents energy of argon or oxygen before collision (300eV), m_(A) represents mass of argon or oxygen, ν_(A) represents thespeed of argon or oxygen before collision, ν′_(A) represents the speedof argon or oxygen after collision, m_(Ga) represents mass of gallium,ν_(Ga) represents the speed of gallium before collision, and ν′_(Ga)represents the speed of gallium after collision.

E=½m _(A)ν_(A) ²+½m _(A)ν_(Ga) ²  (1)

m _(A)ν_(A) +m _(Ga)ν_(Ga) =m′ _(A)ν′_(A) +m′ _(Ga)ν′_(Ga)  (2)

On the assumption that collision of argon or oxygen is elasticcollision, the relationship among ν_(A), ν′_(A), ν_(Ga), and ν′_(Ga) canbe represented by Formula (3).

ν′_(A)−ν′_(Ga)=−(ν_(A)−ν_(Ga))  (3)

From Formulae (1), (2), and (3), on the assumption that ν_(Ga) is 0, thespeed of gallium ν′_(Ga) after collision of argon or oxygen can berepresented by Formula (4).

$\begin{matrix}{v_{Ga}^{\prime} = {{\frac{\sqrt{m_{A}}}{m_{A} + m_{Ga}} \cdot 2}\sqrt{2E}}} & (4)\end{matrix}$

In Formula (4), mass of argon or oxygen is substituted into m_(A), andthe speeds after collision of the atoms are compared. In the case whereargon and oxygen have the same energy before collision, the speed ofgallium when argon collides with gallium was found to be 1.24 times thespeed of gallium when oxygen collides with gallium. Thus, the energy ofgallium when argon collides with gallium is higher than the energy ofgallium when oxygen collides with gallium by the square of the speed.

The speed (energy) of gallium after collision when argon collides withgallium is found to be higher than the speed (energy) of gallium aftercollision when oxygen collides with gallium. Accordingly, a crack isconsidered to be formed at a deeper position in the case where argoncollides with gallium than in the case where oxygen collides withgallium.

The above calculation shows that when sputtering is performed using atarget including the InGaZnO₄ crystal having a homologous structure,separation occurs from the cleavage plane to form a pellet. On the otherhand, even when sputtering is performed on a region having anotherstructure of a target without the cleavage plane, a pellet is notformed, and a sputtered particle with an atomic-level size that isminuter than a pellet is formed. Because the sputtered particle issmaller than the pellet, the sputtered particle is thought to be removedthrough a vacuum pump connected to a sputtering apparatus. Therefore, amodel in which particles with a variety of sizes and shapes fly to asubstrate and are deposited hardly applies to the case where sputteringis performed using a target including the InGaZnO₄ crystal having ahomologous structure. The model in FIG. 43A where sputtered pellets aredeposited to form a CAAC-OS is a reasonable model.

The CAAC-OS deposited in this manner has density substantially equal tothat of a single crystal OS. For example, the density of the singlecrystal OS having a homologous structure of InGaZnO₄ is 6.36 g/cm³, andthe density of the CAAC-OS having substantially the same atomic ratio isapproximately 6.3 g/cm³.

FIGS. 49A and 49B show atomic order of cross sections of an In—Ga—Znoxide (see FIG. 49A) that is a CAAC-OS deposited by sputtering and atarget thereof (see FIG. 49B). For observation of atomic order, ahigh-angle annular dark field scanning transmission electron microscopy(HAADF-STEM) is used. In the case of observation by HAADF-STEM, theintensity of an image of each atom is proportional to the square of itsatomic number. Thus, Zn (atomic number: 30) and Ga (atomic number: 31),whose atomic numbers are close to each other, are hardly distinguishedfrom each other. A Hitachi scanning transmission electron microscopeHD-2700 is used for the HAADF-STEM.

When FIG. 49A and FIG. 49B are compared, it is found that the CAAC-OSand the target each have a homologous structure and atomic order in theCAAC-OS correspond to that in the target. Thus, as illustrated in thedeposition model in FIG. 43A, the crystal structure of the target istransferred, so that a CAAC-OS is deposited.

<Oxide Semiconductor Film and Oxide Conductor Film>

The temperature dependence of resistivity of a film formed with an oxidesemiconductor (hereinafter referred to as an oxide semiconductor film(OS)) and that of a film formed with an oxide conductor (hereinafterreferred to as an oxide conductor film (OC)), such as the oxidesemiconductor films 19 b and 155 b having conductivity, is describedwith reference to FIG. 50. In FIG. 50, the horizontal axis representsmeasurement temperature, and the vertical axis represents resistivity.Measurement results of the oxide semiconductor film (OS) are plotted ascircles, and measurement results of the oxide conductor film (OC) areplotted as squares.

Note that a sample including the oxide semiconductor film (OS) wasprepared by forming a 35-nm-thick In—Ga—Zn oxide film over a glasssubstrate by a sputtering method using a sputtering target with anatomic ratio of In:Ga:Zn=1:1:1.2, forming a 20-nm-thick In—Ga—Zn oxidefilm over the 35-nm-thick In—Ga—Zn oxide film by a sputtering methodusing a sputtering target with an atomic ratio of In:Ga:Zn=1:4:5,performing heat treatment at 450° C. in a nitrogen atmosphere and thenperforming heat treatment at 450° C. in an atmosphere of a mixed gas ofnitrogen and oxygen, and forming a silicon oxynitride film over theoxide films by a plasma CVD method.

A sample including the oxide conductor film (OC) is prepared by forminga 100-nm-thick In—Ga—Zn oxide film over a glass substrate by asputtering method using a sputtering target with an atomic ratio ofIn:Ga:Zn=1:1:1, performing heat treatment at 450° C. in a nitrogenatmosphere and then performing heat treatment at 450° C. in anatmosphere of a mixed gas of nitrogen and oxygen, and forming a siliconnitride film over the oxide film by a plasma CVD method.

As can be seen from FIG. 50, the temperature dependence of resistivityof the oxide conductor film (OC) is lower than the temperaturedependence of resistivity of the oxide semiconductor film (OS).Typically, the range of variation of resistivity of the oxide conductorfilm (OC) at temperatures from 80 K to 290 K is from more than −20% toless than +20%. Alternatively, the range of variation of resistivity attemperatures from 150 K to 250 K is from more than −10% to less than+10%. In other words, the oxide conductor is a degenerate semiconductorand it is suggested that the conduction band edge agrees with orsubstantially agrees with the Fermi level. Therefore, the oxideconductor film (OC) can be used for a resistor, an electrode of acapacitor, a pixel electrode, a common electrode, a wiring, or the like.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 10

In this embodiment, structural examples of electronic devices each usinga display device of one embodiment of the present invention will bedescribed. In addition, in this embodiment, a display module using adisplay device of one embodiment of the present invention will bedescribed with reference to FIG. 51.

In a display module 8000 in FIG. 51, a touch panel 8004 connected to anFPC 8003, a display panel 8006 connected to an FPC 8005, a backlightunit 8007, a frame 8009, a printed board 8010, and a battery 8011 areprovided between an upper cover 8001 and a lower cover 8002. Note thatthe backlight unit 8007, the battery 8011, the touch panel 8004, and thelike are not provided in some cases.

The display device of one embodiment of the present invention can beused for the display panel 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and may be formed so as to overlap with the display panel8006. A counter substrate (sealing substrate) of the display panel 8006can have a touch panel function. A photosensor may be provided in eachpixel of the display panel 8006 to form an optical touch panel. Anelectrode for a touch sensor may be provided in each pixel of thedisplay panel 8006 so that a capacitive touch panel is obtained.

The backlight unit 8007 includes a light source 8008. The light source8008 may be provided at an end portion of the backlight unit 8007 and alight diffusing plate may be used.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 can function asa radiator plate too.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 52A to 52E are each an external view of an electronic deviceincluding a display device of one embodiment of the present invention.

Examples of electronic devices are a television set (also referred to asa television or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.

FIG. 52A illustrates a portable information terminal including a mainbody 1001, a housing 1002, display portions 1003 a and 1003 b, and thelike. The display portion 1003 b is a touch panel. By touching akeyboard button 1004 displayed on the display portion 1003 b, a screencan be operated, and text can be input. It is needless to say that thedisplay portion 1003 a may be a touch panel. A liquid crystal panel oran organic light-emitting panel is fabricated using any of thetransistors described in the above embodiments as a switching elementand used in the display portion 1003 a or 1003 b, whereby a highlyreliable portable information terminal can be provided.

The portable information terminal illustrated in FIG. 52A can have afunction of displaying a variety of information (e.g., a still image, amoving image, and a text image); a function of displaying a calendar, adate, the time, and the like on the display portion; a function ofoperating or editing the information displayed on the display portion; afunction of controlling processing by various kinds of software(programs); and the like. Furthermore, an external connection terminal(an earphone terminal, a USB terminal, or the like), a recording mediuminsertion portion, and the like may be provided on the back surface orthe side surface of the housing.

The portable information terminal illustrated in FIG. 52A may transmitand receive data wirelessly. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an e-bookserver.

FIG. 52B illustrates a portable music player including, in a main body1021, a display portion 1023, a fixing portion 1022 with which theportable music player can be worn on the ear, a speaker, an operationbutton 1024, an external memory slot 1025, and the like. A liquidcrystal panel or an organic light-emitting panel is fabricated using anyof the transistors described in the above embodiments as a switchingelement and used in the display portion 1023, whereby a highly reliableportable music player can be provided.

Furthermore, when the portable music player illustrated in FIG. 52B hasan antenna, a microphone function, or a wireless communication functionand is used with a mobile phone, a user can talk on the phone wirelesslyin a hands-free way while driving a car or the like.

FIG. 52C illustrates a mobile phone, which includes two housings, ahousing 1030 and a housing 1031. The housing 1031 includes a displaypanel 1032, a speaker 1033, a microphone 1034, a pointing device 1036, acamera 1037, an external connection terminal 1038, and the like. Thehousing 1030 is provided with a solar cell 1040 for charging the mobilephone, an external memory slot 1041, and the like. In addition, anantenna is incorporated in the housing 1031. Any of the transistorsdescribed in the above embodiments is used in the display panel 1032,whereby a highly reliable mobile phone can be provided.

Furthermore, the display panel 1032 includes a touch panel. A pluralityof operation keys 1035 that are displayed as images are indicated bydotted lines in FIG. 52C. Note that a boosting circuit by which voltageoutput from the solar cell 1040 is increased to be sufficiently high foreach circuit is also included.

In the display panel 1032, the direction of display is changed asappropriate depending on the application mode. In addition, the mobilephone has the camera 1037 and the display panel 1032 on the same surfaceside, and thus it can be used as a video phone. The speaker 1033 and themicrophone 1034 can be used for videophone calls, recording, and playingsound, etc., as well as voice calls. Moreover, the housings 1030 and1031 in a state where they are developed as illustrated in FIG. 52C canshift, to a state where one is lapped over the other by sliding.Therefore, the size of the mobile phone can be reduced, which makes themobile phone suitable for being carried around.

The external connection terminal 1038 can be connected to an AC adaptorand a variety of cables such as a USB cable, whereby charging and datacommunication with a personal computer or the like are possible.Furthermore by inserting a recording medium into the external memoryslot 1041, a larger amount of data can be stored and moved.

In addition, in addition to the above functions, an infraredcommunication function, a television reception function, or the like maybe provided.

FIG. 52D illustrates an example of a television set. In a television set1050, a display portion 1053 is incorporated in a housing 1051. Imagescan be displayed on the display portion 1053. Moreover, a CPU isincorporated in a stand 1055 supporting the housing 1051. Any of thetransistors described in the above embodiments is used in the displayportion 1053 and the CPU, whereby the television set 1050 can have highreliability.

The television set 1050 can be operated with an operation switch of thehousing 1051 or a separate remote controller. In addition, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 1050 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

Furthermore, the television set 1050 is provided with an externalconnection terminal 1054, a storage medium recording and reproducingportion 1052, and an external memory slot. The external connectionterminal 1054 can be connected to various types of cables such as a USBcable, and data communication with a personal computer or the like ispossible. A disk storage medium is inserted into the storage mediumrecording and reproducing portion 1052, and reading data stored in thestorage medium and writing data to the storage medium can be performed.In addition, an image, a video, or the like stored as data in anexternal memory 1056 inserted into the external memory slot can bedisplayed on the display portion 1053.

Furthermore, in the case where the off-state leakage current of thetransistor described in the above embodiments is extremely small, whenthe transistor is used in the external memory 1056 or the CPU, thetelevision set 1050 can have high reliability and sufficiently reducedpower consumption.

The portable information terminal illustrated in FIG. 52E includes ahousing 1101 and a display panel 1110 which is provided so that an imagecan be displayed on a surface of the housing 1101.

The housing 1101 has a top surface, a rear surface, a first sidesurface, a second side surface in contact with the first side surface, athird side surface opposite to the first side surface, and a fourth sidesurface opposite to the second side surface.

The display panel 1110 includes a first display region 1111 overlappingwith the top surface of the housing 1101, a second display region 1112overlapping with one of the side surfaces of the housing 1101, a thirddisplay region 1113 overlapping with another one of the side surfaces ofthe housing 1101, and a fourth display region 1114 opposite to thesecond display region 1112.

Among the four side surfaces of the housing 1101, at least a regionoverlapping with the display panel 1110 preferably has a curved surface.For example, it is preferable that there be no corner portion betweenthe top surface and the side surface and between the side surface andthe rear surface, and that these surfaces form a continuous surface.Furthermore, the side surface is preferably a curved surface such thatthe inclination of a tangent line is continuous from the top surface tothe rear surface of the housing 1101.

In addition to the display panel 1110, a hardware button, an externalconnection terminal, and the like may be provided on the surface of thehousing 1101. It is preferable that a touch sensor be provided at aposition overlapping with the display panel 1110, specifically, inregions overlapping with the display regions.

With the portable information terminal in FIG. 52E, display can beperformed not only on a surface parallel to the top surface of thehousing but also on a side surface of the housing. In particular, adisplay region is preferably provided along two or more side surfaces ofthe housing because the variety of display is further increased.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Example 1

In this example, a cross-sectional shape of a stacked-layer structure ofan oxide semiconductor film, a conductive film, and an insulating filmwas observed. In addition, composition of metal elements in theconductive film was analyzed. Details of samples manufactured in thisexample are described below.

<Sample A1>

First, a substrate was prepared. As the substrate, a glass substrate wasused. Then, an insulating film 601 was deposited over the substrate.

As the insulating film 601, a 50-nm-thick silicon nitride film, a300-nm-thick silicon nitride film, a 50-nm-thick silicon nitride film,and a 50-nm-thick silicon oxynitride film were successively formed in aPECVD apparatus.

Next, a multilayer film 603 was formed over the insulating film 601. Inthe multilayer film 603, a 35-nm-thick first IGZO film, a 10-nm-thicksecond IGZO film, and a 20-nm-thick IGO film are stacked.

A formation method of the multilayer film 603 is described below. The35-nm-thick first IGZO film was formed under the following conditions:the substrate temperature was 300° C.; a metal oxide target(In:Ga:Zn=1:1:1 [atomic ratio]) was used as a sputtering target; oxygenof 33 vol % (diluted with argon) was supplied as a sputtering gas into atreatment chamber of a sputtering apparatus; the pressure in thetreatment chamber was controlled to 0.4 Pa; and a power of 200 W wassupplied. Then, the 10-nm-thick second IGZO film was formed under thefollowing conditions: the substrate temperature was 200° C.; a metaloxide target (In:Ga:Zn=1:3:6 [atomic ratio]) was used as a sputteringtarget; oxygen of 33 vol % (diluted with argon) was supplied as asputtering gas into a treatment chamber of a sputtering apparatus; thepressure in the treatment chamber was controlled to 0.4 Pa; and a powerof 200 W was supplied. Then, the 20-nm-thick IGO film was formed underthe following conditions: the substrate temperature was 170° C.; a metaloxide target (In:Ga=7:93 [atomic ratio]) was used as a sputteringtarget; oxygen of 75 vol % (diluted with argon) was supplied as asputtering gas into a treatment chamber of a sputtering apparatus; thepressure in the treatment chamber was controlled to 0.4 Pa; and a powerof 200 W was supplied. Next, a mask was formed over the first IGZO film,the second IGZO film, and the IGO film through a photolithographyprocess and etching treatment was performed, whereby the multilayer film603 was formed.

After that, the mask was removed.

Next, heat treatment was performed at 450° C. in a nitrogen atmospherefor one hour, and after that, another heat treatment was performed in amixed gas atmosphere of oxygen and nitrogen at 450° C. for one hour.

Then, a conductive film 605 was formed over the multilayer film 603. Inthe conductive film 605, a 30-nm-thick first Cu—Mn alloy film, a200-nm-thick Cu film, and a 100-nm-thick second Cu—Mn alloy film arestacked.

A formation method of the conductive film 605 is described below. Thefirst Cu—Mn alloy film was formed by a sputtering method under thefollowing conditions: the substrate temperature was room temperature; anAr gas at a flow rate of 100 sccm was supplied to a treatment chamber;the pressure in the treatment chamber was controlled to 0.4 Pa; and apower of 2000 W was supplied to a target with a direct current (DC)power source. Note that the composition of the target was Cu:Mn=90:10[atomic %]. Then, the Cu film was formed by a sputtering method underthe following conditions: the substrate temperature was 100° C.; an Argas at a flow rate of 75 sccm was supplied to a treatment chamber; thepressure in the treatment chamber was controlled to 1.0 Pa; and a powerof 15000 W was supplied to a target with a direct current (DC) powersource. Then, the second Cu—Mn alloy film was formed under theconditions similar to those of the first Cu—Mn alloy film. Next, aresist mask was formed over the second Cu—Mn alloy film, an etchant wasapplied over the resist mask, and wet etching treatment was performed,whereby the conductive film 605 was formed. As the etchant, an etchantincluding an organic acid solution and hydrogen peroxide water was used.

After that, the mask was removed.

Next, an insulating film 607 was formed over the conductive film 605. Asthe insulating film 607, a 50-nm-thick silicon oxynitride film and a400-nm-thick silicon oxynitride film were successively formed in a PECVDapparatus in vacuum.

Next, heat treatment was performed at 350° C. in a mixed gas atmosphereof oxygen and nitrogen for one hour.

Through the above process, Sample A1 was formed.

Next, a cross section of Sample A1 was observed by a scanningtransmission electron microscope (STEM). FIG. 53A shows across-sectional observation image of Sample A1. Note that the image inFIG. 53A is a phase contrast image (TE image).

From the result of the cross-sectional observation image of FIG. 53A, itis observed that the conductive film 605 of Sample A1 formed in thisexample can have a favorable cross-sectional shape over the multilayerfilm 603.

Next, in regions (1), (2), and (3) in FIG. 53A, energy dispersive x-rayspectroscopy (EDX) was performed. FIG. 53B shows the composition of Cuand Mn obtained by EDX analysis. FIG. 53B indicates that Mn is notdetected inside the Cu film ((1) in FIG. 53A), and on the other hand, Mnof 2 atoms % to 4 atoms % is detected on the sidewall of the Cu film((2) in FIG. 53A).

REFERENCE NUMERALS

11: substrate, 12: conductive film, 13: conductive film, 14: gateinsulating film, 15: nitride insulating film, 16: oxide insulating film,17: oxide insulating film, 18: oxide semiconductor film, 19 a: oxidesemiconductor film, 19 b: oxide semiconductor film, 19 c: oxidesemiconductor film, 19 d: oxide semiconductor film, 19 f: oxidesemiconductor film, 19 g: oxide semiconductor film, 20: conductive film,20_1: conductive film, 20_2: conductive film, 21 a: conductive film, 21a_1: conductive film, 21 a_2: conductive film, 21 b: conductive film, 21b_1: conductive film, 21 b_2: conductive film, 21 c: conductive film, 21c_1: conductive film, 21 c_2: conductive film, 21 d: conductive film, 21d_1: conductive film, 21 d_2: conductive film, 21 e: conductive film, 21e_1: conductive film, 21 e_2: conductive film, 21 f: conductive film, 21f_1: conductive film, 21 f_2: conductive film, 21 g: conductive film,22: oxide insulating film, 23: oxide insulating film, 24: oxideinsulating film, 25: oxide insulating film, 26: nitride insulating film,27: nitride insulating film, 28: conductive film, 29: common electrode,29 b: conductive film, 29 c: conductive film, 29 d: conductive film, 30:inorganic insulating film, 30 a: inorganic insulating film, 31: organicinsulating film, 31 a: organic resin film, 33: alignment film, 37 a:multilayer film, 37 b: multilayer film, 38 a: multilayer film, 38 b:multilayer film, 39 a: oxide semiconductor film, 39 b: oxidesemiconductor film, 40: opening, 41: opening, 41 a: opening, 49 a: oxidesemiconductor film, 49 b: oxide semiconductor film, 101: pixel portion,102: transistor, 102 a: transistor, 102 b: transistor, 102 c:transistor, 102 d: transistor, 102 e: transistor, 103: pixel, 103 a:pixel, 103 b: pixel, 103 c: pixel, 104: scan line driver circuit, 105:capacitor, 105 a: capacitor, 105 b: capacitor, 105 c: capacitor, 106:signal line driver circuit, 107: scan line, 109: signal line, 115:capacitor line, 121: liquid crystal element, 131: light-emittingelement, 133: transistor, 135: transistor, 137: wiring, 139: wiring,141: wiring, 151: substrate, 153: insulating film, 153 a: insulatingfilm, 154: rare gas, 155: oxide semiconductor film, 155 a: oxidesemiconductor film, 155 b: oxide semiconductor film, 155 c: oxidesemiconductor film, 156: coating film, 156 a: coating film, 156 b:coating film, 156 c: coating film, 157: insulating film, 157 a:insulating film, 159: conductive film, 159 a: conductive film, 159 b:conductive film, 159 c: conductive film, 160 a: resistor, 160 b:resistor, 160 c: resistor, 160 d: resistor, 160 e: capacitor, 160 f:capacitor, 160 g: resistor, 160 h: resistor, 160 i: resistor, 161:conductive film, 161 a: conductive film, 161 b: conductive film, 161 c:conductive film, 162: conductive film, 162 a: conductive film, 162 b:conductive film, 162 c: conductive film, 163: conductive film, 163 a:conductive film, 163 b: conductive film, 163 c: conductive film, 164:conductive film, 164 a: conductive film, 164 b: conductive film, 164 c:conductive film, 170 a: protection circuit, 170 b: protection circuit,171: wiring, 172: wiring, 173: resistor, 173 a: resistor, 173 b:resistor, 173 c: resistor, 174: transistor, 174 a: transistor, 174 b:transistor, 174 c: transistor, 174 d: transistor, 175: wiring, 176:wiring, 177: wiring, 180 a: capacitor, 180 b: capacitor, 180 c:capacitor, 180 d: capacitor, 180 e: capacitor, 180 f: capacitor, 180 g:capacitor, 181: conductive film, 306: insulating film, 320: liquidcrystal layer, 322: liquid crystal element, 342: substrate, 344:light-blocking film, 346: coloring film, 348: insulating film, 350:conductive film, 352: alignment film, 370 a: light-emitting element, 370b: light-emitting element, 371: insulating film, 373: EL layer, 375:conductive film, 601: insulating film, 603: multilayer film, 605:conductive film, 607: insulating film, 609: metal oxide film, 612:conductive film, 1001: main body, 1002: housing, 1003 a: displayportion, 1003 b: display portion, 1004: keyboard button, 1021: mainbody, 1022: fixing portion, 1023: display portion, 1024: operationbutton, 1025: external memory slot, 1030: housing, 1031: housing, 1032:display panel, 1033: speaker, 1034: microphone, 1035: operation key,1036: pointing device, 1037: camera, 1038: external connection terminal,1040: solar cell, 1041: external memory slot, 1050: television set,1051: housing, 1052: reproducing portion, 1053: display portion, 1054:external connection terminal, 1055: stand, 1056: external memory, 1101:housing, 1110: display panel, 1111: display region, 1112: displayregion, 1113: display region, 1114: display region, 5100: pellet, 5100a: pellet, 5100 b: pellet, 5101: ion, 5102: zinc oxide layer, 5103:particle, 5105 a: pellet, 5105 a 1: region, 5105 a 2: pellet, 5105 b:pellet, 5105 c: pellet, 5105 d: pellet, 5105 d 1: region, 5105 e:pellet, 5120: substrate, 5130: target, 5161: region, 8000: displaymodule, 8001: upper cover, 8002: lower cover, 8003: FPC, 8004: touchpanel, 8005: FPC, 8006: display panel, 8007: backlight unit, 8008: lightsource, 8009: frame, 8010: printed board, 8011: battery.

This application is based on Japanese Patent Application serial no.2013-248284 filed with Japan Patent Office on Nov. 29, 2013 and JapanesePatent Application serial no. 2014-038615 filed with Japan Patent Officeon Feb. 28, 2014, the entire contents of which are hereby incorporatedby reference.

1. A semiconductor device comprising: an insulating layer; a first oxidesemiconductor film having conductivity; a transistor including a secondoxide semiconductor film; and a first conductive film in contact withthe first oxide semiconductor film and the second oxide semiconductorfilm, wherein the first conductive film includes a Cu—X alloy film and aCu film over the Cu—X alloy film, wherein X is Mn, Ni, Cr, Fe, Co, Mo,Ta, or Ti, wherein the first oxide semiconductor film has conductivity,wherein a hydrogen concentration of the first oxide semiconductor filmis higher than or equal to 8×10¹⁹ atoms/cm³, wherein a hydrogenconcentration of the second oxide semiconductor film is lower than orequal to 5×10¹⁹ atoms/cm³, wherein a resistivity of the first oxidesemiconductor film is higher than or equal to 1×10⁻⁸ times and lowerthan 1×10⁻¹ times a resistivity of the second oxide semiconductor film,wherein the first oxide semiconductor film having conductivity is overand in contact with the insulating layer, and wherein the insulatinglayer includes hydrogen.
 2. The semiconductor device according to claim1, wherein the resistivity of the first oxide semiconductor film ishigher than or equal to 1×10⁻³ Ωcm and lower than 1×10⁴Ωcm.
 3. Thesemiconductor device according to claim 1, wherein the first conductivefilm includes a Cu—Mn alloy film.
 4. The semiconductor device accordingto claim 3, wherein a part of the Cu film is covered with a filmincluding manganese oxide, and wherein the part of the Cu film is incontact with the film including manganese oxide.
 5. The semiconductordevice according to claim 1, wherein the first oxide semiconductor filmincludes hydrogen and an oxygen vacancy, and wherein the hydrogen islocated in the oxygen vacancy.
 6. The semiconductor device according toclaim 1, further comprising a capacitor, wherein the capacitor includesthe first oxide semiconductor film, the first conductive film, aninsulating film, and a second conductive film, wherein the insulatingfilm is over the first oxide semiconductor film and the first conductivefilm, and wherein the second conductive film is over the insulating filmand overlaps with the first oxide semiconductor film.
 7. Thesemiconductor device according to claim 6, wherein the insulating filmis a nitride insulating film.
 8. The semiconductor device according toclaim 1, wherein the hydrogen concentration of the first oxidesemiconductor film having conductivity is higher than or equal to 5×10²⁰atoms/cm³.